Their encryption is based on PGP and therefore only message contents are "E2E" encrypted. Subject, From, To, etc. are not. These fields contain most of the information already. For example, Amazon puts the name of the ordered item in the subject line, so they can still see what you ordered.
And I'm putting "E2E" in quotes because if the sender does not send encrypted emails, then they can read the full content at delivery time, obviously. They immediately encrypt them with your public key and they claim that they discard the unencrypted version after that but there is no way we can verify that.
Long story short: you still have to trust your email provider after all. If I'd want to switch away from Google, I'd probably switch to some "normal" email provider (Fastmail, Apple, etc.). The benefits of "E2E" encryption for email are questionable and the drawbacks huge (for example search is very limited). But competition is good and I'm glad they are advancing.