It doesn't. This guy is saying that a site that requires login and has ToS allowing them to scrape your stuff and use it for themselves is better than a site where your art is publicly available. Which is a pretty dumb argument IMO, because a) that ToS clause presumably does something (otherwise why would they have it?) b) anyone who would scrape your art from bluesky can just pay Twitter a couple of cents to get it instead (because that clause allows that) c) having only Elon's AI company able to use your stuff isn't actually any better than having every AI company able to use your stuff. Arguably it's worse.
I'm no fan of Bluesky but this is a dumb argument. If you want to post your stuff somewhere public, it's going to be public, and if you want to post it publicly but not give permission for AI training, that's not in itself an insane thing that's necessarily failed.