Eh, if you don't trust your masto instance admin to not read your DMs do you really trust them to not break the "your password never leaves the client" guarantees that protonmail for eg. promises?

This is the thing about this argument: Either you trust your instance admins or not. If they promise you e2e and you don't trust them, you should rightly look at that as snakeoil.

This is meaningless if you don't trust the site admins, and the reason to use e2ee in the first place is to avoid trusting the site admins. All it takes is for them to serve you different JavaScript one time that exfiltrates your messages, and I guarantee you'll never notice.