[teleproto]

But are they open source yet?

Sure, you can find the web client sources. How about the server, and the mobile apps? The website makes a big deal of them being open source after all ( https://protonmail.com/blog/protonmail-open-source/ ).

[mi100hael]

Mobile apps should be open source, but server doesn't really matter. It's just some boring plumbing shuffling around a bunch of encrypted blobs.

[danenania]

There's also no way to verify that a server is really running the same code that was open-sourced.