As I understand it, snap the package format is not proprietary. Its as open source as say flatpak. What is proprietary is Canonical official snap store, and they patch their version of snap to only use that store. It'd be the same as flatpak being tied to only flathub.

Of course that goes against the spirit of FOSS, but there's a bit more nuance there than simply saying "snaps are proprietary".

I don't really understand why this is such a big problem. You don't have to use snaps.