Why do you think the fact they don't use reddit matters?

On a side note: I think they managed to come up with a good UI targeted at the use case of asking questions to one person.

There's no followup, which is an essential part of asking questions, both to deal with evasion but also to clarify what you were asking. If they don't answer your question, you have no recourse, no call out, no opportunity to say: "Obviously you don't know the exact number, but is it ten or a hundred or a thousand?"

"both to deal with evasion but also to clarify what you were asking"

Very true. They would need to add comments and allow the interviewee to edit his/her answers.

Comments are coming; answers can already be edited

Answers like "We're a surprisingly small team" would never fly on Reddit