I believe you’re completely right here, except that snapd can be configured to point to another store, though it’s not very well documented… I did find the piece of information once :).
But the thing is that the client still only supports one app backing site at a time. So if you pick another one, you lose visibility to the other store. I doubt even updates work as they should.
So it’s really about building technology that is geared towards centralized control, whereas basically anyone can host flatpak packages and give ref links to them.