Corollary: if you feel that something is wrong, and you need to speak out against it, be clear what you're speaking out against. Maybe what the author was wrong about is simply how people would perceive their post. That's a serious failure, assuming the goal was to communicate an idea, but it doesn't invalidate the idea.

Yes, there's a risk of permitting motte-and-bailey if you allow them to say "no, that's not what I was talking about at all". However, one such retreat does not constitute evidence that they will return to their motte and that the supposed bailey really isn't their intended ground. Regardless of whether it's a true misunderstanding of position or not, though, they have not refuted your counterpoint; that is an acknowledgement that you are right..

It costs you very little, but potentially informs everybody involved, to identify the position you're arguing against. It's worth doing.