We can't reliably detect AI text, and they are getting better at producing plausible humanlike text (albeit still with flaws and mistakes). I do expect them to become impossible for humans to detect when the companies put effort into that (so far they've been focussed on other domains like dev or maths).

I'm afraid your dystopian future where the zone is flooded with shit and nobody can trust anything seems eminently plausible, partly because the internet was built without identity.

I think to solve this you'd have to have strong proofs of identity locked to biometric identifiers. We'll probably need that anyway for other reasons.

It's not a trivial problem. Websites like this could require webauthn tied to a hardware identifier of their choice (like a fingerprint or faceid) - I'm not sure if webauthn provides that level of detail to providers (which method was used), and how secure that attestation is. Of course that's not going to be popular but it might be the lesser of two evils.