I think that's his point: it's difficult to differentiate things that are likely wrong/false and things that have been verified by the community. Flags could help that.
I prefer simply relying on the comments, personally. In the comments you can see who's knowledgeable about the topic and often they'll explain what's wrong with the article. Votes and flagging can't do that.
Flags are a binary mechanism just like voting, and as such are vulnerable to precisely the same epistemic problems. As you say, comments offer a way of demonstrating expertise, not merely asserting it.
For this reason someone interested in a disputed topic such as this will either have to read the comments, in the hope of discovering the main lines of argument and improving the likelihood that the picture they form is correct, or suspend their judgement until such a time as better evidence (in this case, peer review of the claimed proof) is available.
I think that's his point: it's difficult to differentiate things that are likely wrong/false and things that have been verified by the community. Flags could help that.
I prefer simply relying on the comments, personally. In the comments you can see who's knowledgeable about the topic and often they'll explain what's wrong with the article. Votes and flagging can't do that.