The trust doesn’t even have to be in the encryption, they could very well use the same signal protocol. They would only need a copy of the keys you are using and you wouldn’t even know… That’s the problem with closed source programs, there is no certainty that its not happening (and I’m not saying it is, I can’t prove it, obviously, but the doubt remains, we need to trust these companies not to screw us over and they don’t really have the best track record in that…)