I would say Telegram is communicating their level of encryption pretty good ("client-to-client" and "client-to-server" is a good way to avoid the ambiguity of e2e).

https://telegram.org/faq?setln=en#q-so-how-do-you-encrypt-da...

Please cite where Signal is not e2ee.

The problem is that you have to trust that they'll stay that way, and we have no way of proving that the app that runs on your phone comes from the same source that they publish.

That's a completely different problem than marketers co-opting the term "end-to-end encryption".