So what is stopping us from building native Linux client for Proton Drive, as Proton is clearly not interested in doing that?

Nothing. That's the point of making their repos available. They simply don't view the Linux market as sufficiently large enough to justify handling this internally.

Oh wow. Wouldn't this make it easy for someone to just use/fork the code here to create a competitor?

Only the clients source code are available, so we know what data is encrypted before sending to server. The server side is closed source.

Yeah, but why would anyone use the competitor? Proton’s biggest advantages are its security and privacy. It’s hard to back that up with code and even more with plain words.

Having their code be open source is a very important step in maintaining that image, and that image is not something a competitor can just plagiarize.

In addition to what others have said, the license is GPL3, so a competitor would legally need to open source their app too.