One advantage is that if it's labelled as AI, we don't need to have a conversation in the comments about whether it's AI or not.

Only if you view the label as accurate, which you really, really should not.

If an account claims to be a bot, I don't see much downside in assuming it's true?

I'm referring to the tag that Youtube is going to assign automatically.

I wish HN would do the same for submissions.