The Accepted Answer Violates SE's Abusive behavior policy
The stack exchange abusive behavior policy states:
To ensure that all users feel safe and welcome, we do not allow behaviors or content that cause or contribute to an atmosphere that excludes, marginalizes, or dehumanizes individuals or communities
... and goes on to define Bullying and Harassment as:
Severe, repeated, or persistent unsolicited conduct, misuse of power or tools, or attacks that target specific users or groups of people in a manner that causes harm. Content that contributes to a hostile or threatening environment, denies a person's expressed gender identity, or invalidates a person's individual experiences in a manner that causes harm.
To understand how this is a violation of the abusive behavior policy, you need to first understand that not all people solve problems the same way. People in the autism spectrum typically suffer from a Mnesic Imbalance limiting them to solving problems using only top-down (concept-before-the-details) or bottom-up (details-before-the-concept) thinking. This does not mean that people with a Mnesic Imbalance are not intelligent, people at both ends of this disorder are both common and very capable thinkers given reasonable accommodation.
By enforcing a policy against High Concept Questions you are saying that to use this platform, you MUST be a bottom up thinker which discriminates against and excludes many people Mnesic Imbalances because they literally can not get to the details until they have figured out the high concept. The high concept is not just an author's choice. Just like the smaller more specific rules, it must be cohesive with reality and/or the reader's expectations to create a believable setting. While some authors are better at building up the high concept from smaller details, this is not universal writing style and the variance in different author's cognitive abilities deserve to be accommodated.
A Few Frame Challenges
There are several common misconceptions in the OP's question that have lead him to believe there is a problem with asking High Concept questions.
Two obvious problems with this kind of question:
There seems to be a misunderstanding here about what "too broad" means. A question can itself be very broad, but still compel a simple and concise answer. High Concepts are by definition ideas that encompass a lot of other ideas, but can itself be treated as a single idea. So, if you ask a broad question about the tree density in a forest, then an answer breaks down every type of forest and all the hundreds of different kinds of trees that live in each one... it's is not really answering the question as asked; so, it's not the OPs fault that the person answering him sees it as "too broad" when a simple statistical average would have answered the question just fine.
Opinion Based means that the OP is asking for your opinion, not that a question might be answered with an opinion. If you do not know HOW to answer a question without just stating an opinion, then you as the person answering should just not try. Leave it open and let the next guy try to figure it out. I've seen countless questions over the years get VtCed as opinion based because someone could only think of opinions to answer with and then go on to be answered with an extremely factual, well researched, and highly rated answer. Opinion Based VtC means that you are expressly asking for an opinion, not that it might be answered with an opinion.
NOT OPIONION BASED: Assume Earth and history as we know it, but in my world, Hitler died from whooping cough as a baby. What is my world like after this change?
OPIONION BASED: In my world, Hitler died from whooping cough as a baby. Is this a good idea for a story?
Nevertheless: in practice they have just asked us to do their work as an author for them.
No they haven't. If they ask for a High Concept, and you can only figure our how to answer it with a list of details, that is your fault, not theirs. If they ask for help with a High Concept, then just address the High Concept. There is still tons of work that comes after that in filling out the details that they can do on thier own.
Solution: High Concept Questions need High Concept Answers
In summery, if you see a question asking for help with a high concept, then answer the question as it is asked and only as it is asked. In your head you might be able to break down thier question into 100 parts and answer them each accordingly, and extrapolate a whole setting from thier one question... but if that's not what was asked, then just don't do it. Only if they expressly ask for all the low level details should you VtC it as being too broad.
Spotting the difference between a Good and Bad High Concept Question
Assume Earth and history as we know it, but in my world, Hitler died from whooping cough as a baby. What is my world like after this change?
This is a good High Concept question because it asks for a vague answer to a specific question. A vague answer in this context means that you are talking in probabilities and generalizations instead of specific details. A vague answer tells you what direction to go in without dictating each step you must take. So a good answer to a question like this might read:
History would mostly have stayed the same because Hitler only highlighted existing social problems. Fascism, Racism, and bitterness about WWI would all happen with or without Hitler. If he did not take power as he did, then someone like him, influenced by the same environment and principles would have likely risen to take his place. So, you might see some minor changes here and there, but you'd still probably see the rise of a powerful and militant fascist Germany in the mid-20th century leading to a similar future as our own.
Here you can see that the question can be answered following facts and evidence without going into a whole cascade of details and similar possible answers.
A bad high High Concept Question is when someone asks for specific details about a topic that is too broad to answer in specific details:
Assume Earth and history as we know it, who would I have to kill as a baby to prevent WWII?
This is too specific for a High-Level Concept because it can be answered too many ways. Millions of people contributed to WWII and killing off any given person or group of people might be able to prevent the war.
Assume Earth and history as we know it, but in my world, Hitler died from whooping cough as a baby. How would each decade from then until now have changed because of it?
This is too specific for a High-Level Concept because it can only be answered with an exhaustive list of ideas and possibilities where each variable splinters off more ideas.