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.