Well snap itself isn’t proprietary, the backend server distributing the snaps is.
Explain how this distinction matters in the real world?
Snap distribution is as much a part of snaps as Snapd.
Who cares that part of it is open source if other parts aren’t?
If Canonical folded Snap could be taken over by others who could build new server software for it, either from scratch or based off the other projects to develop alternative servers for it, and modify snap to accept multiple repos like that. That’s the difference, also just being able to fork snap like that. Though the fact it hasn’t been done says something about how many real snap enthusiasts there actually are.
If Canonical folded, someone else could come along and reinvent everything on the server side. And that makes it Open Source?