It does sound condescending. I think the sentiment is important though. Asking someone to be specific can help them think clearly. What’s a nicer way to do that?
I think "Who, specifically, claims that [...]?" comes off as less condescending than "Who claims that [...]? Be specific." just by virtue of the latter using imperative language, which triggers a reflexive "you're not the boss of me" reaction.
The message is clear in both cases. It's easier to put aside these irrational reflexive reactions and think about
whatever worth can be derived from the message than it is to carefully manage the emotions of varied readers whom you don't know. This is different from bring overtly inflammatory, although the lines for this are subjective.
