What many people miss about the dead internet theory is that AI is only a small part of it.
Web pages for brands, businesses, companies, governments, NGOs and the like are part of the dead internet. Propaganda, paid for posts, shills, psyops, astroturf shit and current thing posts are also part of the dead internet.
The point that the dead internet theory makes is that the majority of "content" you interact with isn't being posted by "real" people. If someone posts something because he's being paid to do it, or like in China, intimidated into doing it, or doing it out of complacency, they no longer count as real people who have come to discuss stuff on the internet. It doesn't matter what they look like, where they're from or even what they post, as long as they do it out of their own free will.
This is why the internet feels so artificial and that makes it dead.