It's self custodial.
Discussion
Forgive the pedantry, but wouldn’t it not be fully self-custodial if you are giving or storing your keys with another party? If they can sign transactions for you they can also sign transactions to steal from you
I guess they don't have access to the wallet keys in any way.
But I've not checked the source code.
It is a soft lie. They can always provide a different version of the website that captures your unlocking passwords when your IP address is interesting to governments.
Even messages shared here privately are more private than Proton.