Earlier quoted context omitted.
Website authors opting in to letting another server serve content "as" their own domain isn't anything new: it's what CDNs do. The Signed Exchanges standard is actually a huge step up because the website authors get to cryptographically sign their content. With normal CDNs, if the CDN is malicious or gets hacked, the CDN is free to serve modified content on the original domain.
I can already smell the fake news from hacked keys, and authoritarians demanding keys from media sites. Congrats, we found just another way to break the resilience of decentralisation
If an authoritarian can man-in-the-middle network connections and demand keys, it would be much simpler to just MITM the sites without doing anything with Signed Exchanges.