That's interesting and not actually all that intuitive (to me at least). I could see how even in a hypothetical scenario where there is only 1 relay, using key pairs could still prove a note hasnt been tampered with? Wouldn't that still be valuable?
Discussion
I think the point he's making is if there's 1 relay, it's just twitter, or if you have one you're on of many it's just AP, no need to prove who you are because the source is unique. When there are multiple potential sources that can be amorphous, for people to fetch your messages the architecture needs a way to prove they came from you.
Signatures are useful to prove something wasn't tampered with, yes.
Yea, I understand. I guess I just don't agree with the absolute statement "is only needed because..."