Of course you can access everything through the web on Linux. I really like Proton's web mail interface. Unfortunately, Proton does not have a Linux analog to their windows client that provides automatic file syncing. I think that what the commenter is complaining about.
There is a dedicated Linux client for Proton VPN and in my experience it integrates quite well on Debian-based distributions.
I use PopOS as my daily driver on my desktop. The tiling window management is simply the chef’s kiss.
I’m stoked for Cosmic DE, it’s awesome to see further community adoption of System76’s contribution to Linux.