Obviously you need someone joining the room for the room metadata to be shared between homeservers.

Well then, your assertion that Matrix gives it freely is false.

Not so with Matrix, where a joining homeserver get full retroactive access to all the room metadata since the room’s creation.

This is false, too. Historical event visibility is controlled by a room setting. (And if you don't trust admins of a sensitive room to configure for privacy, then you're going to have bigger problems, no matter what platform it's on.)

Edit: I suppose you might argue that you can bypass this by running your own homeserver and attempting to join the room from it, thereby granting visibility not through joining (as you wrote), but instead through federation with the server you control. The thing is, you can't do it without permission. Room admins can simply deny your join request when they see what server you're on. This might make sense in a particularly sensitive room, for example, just as it would to restrict history visibility.

you really need to stop privacy LARPing

LARPing? I'm not the one stirring up drama with falsehoods and patronizing snark, am I? Farewell.