I don't think the wallet is open source either. I don't know. I just wrote to customer support asking about that.
The wallet is supposedly both non-custodial and open-source.
I prefer a wallet that I do not need email, username and password to login. Just seed phrase or private key is enough to access my coins.
Proton Wallet is a BIP39 compatible wallet that provides users with a 12-word seed phrase. On top of that, there is some sort of master seed, which they call recovery phrase of your Proton account. You can also extend your seed with passphrases and activate 2FA.
If both the sender and receiver use Proton Wallet, you only need the e-mail address of the receiver to send bitcoin. The wallet automatically selects a new and unused bitcoin address that belongs to the receiver in the background. As soon as it gets funded, it gets removed from circulation to never be used again (to avoid address reuse). You can, of course, also send your BTC by manually entering bitcoin addresses of any other wallets and services.