The underwater question is more on-topic than the outrunning question, although both are borderline.
The underwater question is how far down do you have to go so as to survive the nuke. That's a question of setting and how you create the characteristics of your world.
The outrunning question is about a single person's actions and whether they are feasible. The setting is given to us (in particular the distance, which was the previous question). To call this Worldbuilding, we have to basically think of it as a reality check.
Personally, I'm fine with both questions being on-topic. They are both heavily plot-based, but they aren't asking for general suggestions. We are given the plot purely to talk about aspects of the setting. They could both be just this side of the on-/off-topic divide.
The problem with making the underwater question off-topic is that we are essentially being pedantic. Consider the following question:
How far underwater does my city need to be?
In my Earth, there was a nuclear war. Fortunately, some people had moved into underwater cities. I want to locate one near Pearl Harbor, Hawaii. How far underwater does the city need to be to have survived a one megaton hydrogen bomb that hits Pearl Harbor directly? Whatever the depth, I want it to be as close to Pear Harbor as possible. So if you tell me one kilometer, then I want to locate it at the closest one kilometer deep water to Pearl Harbor.
This question seems clearly on-topic to me. It's about aspects of the world rather than the plot. But it's essentially the same question and will have very similar answers. So if we make the other question off-topic but accept this one, we are essentially encouraging people to lie about what their real question is so as to comply with an arbitrary rule boundary.
You can sort of do the same with the outrunning question, but it's more complicated and may require multiple questions. Rather than having one person trying to escape in one car, ask about traveling into the blast crater. When ten kilometers from the center, how does the road look? Are there cars on it? How close to you have to get before fleeing cars couldn't escape? What do they look like? Are the tires still there?
So in my opinion, both questions should be on-topic, although we should guide people to concentrating more on setting and less on plot. I do think that the underwater question is slightly more on-topic than the outrunning question.