Ah OK, I stand corrected, thanks. This must be recent then. They did not have this 4 or 5 months ago when I looked and I remember being quite surprised by this. I was told it was a feature that was on the roadmap. Why is this not listed on security details for the product I wonder?

https://protonmail.com/security-details

First of all encryption which is what a master passphrase gives you is not the same as authentication. These are two different concerns.

Second, how well does that master passphrase plus local local key work for you when you are not on your laptop or when possibly using an untrusted computer?

Are you just trolling? Two factor authentication is not buzzword.