I understand the intent, but this is not phrased well.

prevents your messages being spied on by Signal, but ironically they’re probably one of the most trustworthy actors in this whole chain, so the fact that it’s protected from them, while commendable, is not particularly valuable security

It's extremely valuable security, because most companies, even if they don't want to spy on you might be compelled to by court order. And those companies often think their security is sufficient because they have good intentions, and they expect the government to have good intentions when they're going as far as getting a court order. (I also suspect more court orders are justified than not, but a few bad subpoenas spoil the bunch.) The fact that they physically are unable is quite important.

All your points about how things around that can fail are valid.