DeBuff

[Reece]

Now, it's mentioned that this is the case for defensive super moves or moves that use the Area Shield Proficiency, but I can't find it mentioned anywhere else in the FAQ or in any of the Quick Bar threads.

From the Rules page for Moves, listed under Writing Moves, in the sixth paragraph of the sub-section Additional Writing Notes:

Moves, whether offensive or defensive, may not have benefits against particular elements or character types. This is because due to the nature of our site, some characters may solely utilise a particular elements like flame or electricity, whereas others may use several elements, or original elements that exist only in their own fiction. We don't want people to pick characters based on perceived advantages and disadvantages, or characters who use many elements to have an advantage over those who are specialised. Therefore, these elements are to be regarded as "fluff". If you want to give your move fodder by saying it only works against evil characters or mechanical characters, that's your choice, but it won't improve the move's efficacy or give you any special advantage, because it is purely fodder.

So, as it says here, you can actually make moves which work solely against, let's say, magic users... however, this would not count as a drawback. In other words, you'd need to make the move weak enough that it could be approved even if it affected everyone equally, and then add the RP Fodder weakness, stating that it only worked against specific targets (and making clear that you weren't counting it as a drawback).
That said, people generally don't do that, for obvious reasons.

I mean if anything it's frankly a weakness.

It's supposed to be.