How is it not a Telegram replacement? You can also join a private Telegram group and forward messages from there to other places with the full assurance (provided by Telegram directly) that these messages came from specific people in that group.

In any case, screenshots are enough in 90% of the cases, in other cases -- like if you have a Signal chat and people don't believe screenshots -- you can always do stuff like record a video of the chat screen in your phone or just show your phone directly, I don't know, none of this is impossible to break in the real world. Ultimately you have to trust the people you send your messages to or the people you invite to your secret group.

Also blasting cryptographically signed messages is probably not going to do anything in practice since you'll need a compliant client to even display them -- or you must be a hacker and essentially make your own client, which most people aren't and won't bother.

To me 90% of the benefits of NIP-29 are public groups though, just a way to segregate the conversation. NIP-29 started as just custom relays fit for the specific purpose of chat (as you say, the air reply stuff), but it turns out that that design is flawed in some ways, so the current NIP-29 is slightly better and more portable, more flexible.