It would be a good idea if there could be a banner making note of the hard-science tag when entering an Answer. This is especially true with the influx of newcomers.
I think it requires a new feature, to show a banner when adding this tag. Explain the rules and suggest science-based as a possible alternative.
Likewise, include a banner when opening the Answer editor.
A custom post notice can be created to tag answers that don't follow the rules, as an alternative to flagging it “not an answer” and typing a remembered comment again. This is in the same line as the examples shown in the comment (e.g. citations needed) and others SEs use them for this purpose.
I noticed some tags on the original SO have icons. I found it overly cute and annoying. But, they can be used for good UI design! Use an icon on tags that represent semantic modifiers as opposed to a subject, as these should be noted before answering. E.g. reality-check, science-based vs magic.
And tags that impose strict rules like hard-science, must read (understand) before answering, can have a stronger icon.