the accusation was always made of straw

I expect that's correct, but I'm not sure your justification for it is correct. In particular it seems obviously possible for the following things all to be true:

• A thinks her group is better than others.
• A's thinking this is obvious enough for B to be able to discern it with some confidence.
• A never explicitly says that her group is better than others.

and I think people who say (e.g.) that atheists think they're smarter than everyone else would claim that that's what's happening.

I repeat, I agree that these accusations are usually pretty strawy, but it's a slightly more complicated variety of straw than simply claiming that people have said things they haven't. More specifically, I think the usual situation is something like this:

• A really does think that, to some extent and in some respects, her group is better than others.
• But so does everyone else.
• B imagines that he's discerned unusual or unreasonable opinions of this sort in A.
• But really he hasn't; at most he's picked up on something that he could find anywhere if he chose to look.

[EDITED to add, for clarity:] By "But so does everyone else" I meant that (almost!) everyone thinks that (many of) the groups they belong to are (to some extent and in some respects) better than others. Most of us mostly wouldn't say so; most of us would mostly agree that these differences are statistical only and that there are respects in which are groups are worse too; but, still, on the whole if a person chooses to belong to some group (e.g., Christians or libertarians or effective altruists or whatever) that's partly because they think that group gets right (or at least more right) some things that other groups get wrong (or at least less right).