I don’t think so. It doesn’t use the double ratchet algorithm used in DMs but still the participants of the group formulate a secret key for the group and share this key with every participant using the more secure protocol. The key cannot be read by the server as it is shared using conventional E2EE.

Refer this video from computerphile for more details. Start watching from 5:24. https://youtu.be/Q0_lcKrUdWg

This isn't true at all. It uses Signal Protocol and is end-to-end encrypted.

It is proprietary, so there is no proof they didn't tamper with the code to appear to be working on clients. And chances are against the fragile link in the chain - the user.