If the owner of the standard notes will now be a proton, doesn’t that contradict this principle?

There's no principle... Standard Notes was never about having an open solution or going against the big co. it was about creating something that could be monetized.

Let’s see what Proton does with this, but I personally believe they’ll just integrate it in Proton and further close things even more. The current subscription-based model, docker container and whatnot might disappear as well. Proton is a greedy company that doesn’t like interoperability and likes to add features designed in a way to keep people locked their Web UI and applications.

Standard Notes for self-hosting was already mostly dead due to the obnoxious subscription price, but it is a well designed App with good cross-platform support and I just wish the Joplin guy would take a clue on how to design UIs from them instead of whatever they’re doing now that is ugly and barely usable.