Prediction markets, a specific prediction market, or something related to prediction markets must be a main subject of the question, not just an offhand reference.
The question must be one asked by someone else in the Q&A, not a question posed by CGP Grey themselves. It must be featured in the Q&A video, not just asked in the submission forum. In the event there are multiple videos that are all part of the same conceptual Q&A session, I'll count all of them that are on the main channel.
Related questions
@IsaacKing removes ambiguity. There almost certainly will be a question about PMs if anyone can submit a question; I think your intention with the market is determining whether he responds to one of those questions
@BrendanFinan I mean in the Q&A video itself, not just submitted on Reddit. As far as I'm aware Grey isn't answering questions on the Reddit post, so I don't think that's the actual Q&A. I'll edit the description to be more clear.
(Your change is a functional one, since Grey could mention a question in the video and decline to answer it.)