A Matrix server can force E2E on all messages passing it?

Yes, it can - for instance the Matrix servers that the French government runs do this by default already. We haven't exposed this as a config option and pushed it into mainstream Synapse yet because of https://github.com/vector-im/riot-web/issues/6779, but the tweak for doing it is:

https://github.com/matrix-org/synapse/pull/3426/files

Is this true? As far as I know, even in rooms with m.room.encryption, you can still send normal m.room.messages. Or does the config option you're describing automatically setup room PLs such that m.room.message can't be sent?

technically you can still send normal m.room.messages (or anything else), but any spec-compliant client will refuse to do so in a room where m.room.encryption has been enabled.

That looks like a no given the article has a "Create a new secure room" section where you have to explicitly enable encryption for that specific room.

e2e, by nature, is client specific. So Matrix, as a connectivity glue protocol, has nothing to do with it.

Synapse, the reference server, can handle rooms, and force encryption on the rooms.

p2p is client side. Riot, as reference client, is the one that takes care of this, and, if I get everything right, it is on by default.

Regardless of the truth of your assertions, the article you provided as evidence does not support them. At no point does it demonstrate that a matrix server can force E2E on all communications, that synapse can be configured thus, or that riot can require that configuration.