While Canonical’s particular snap store implementation is closed source, all of the client software as well as the store API are open, and snap isn’t even tied to using snaps from their store. One could easily make a client app that treats snapd much the way apt treats dpkg. (In fact given apt-rpm I think it would probably be feasible to quite literally use apt for that.)

KDE seems to upload their packages to the snap store for Neon, judging from their page.

KDE also maintains most of the flathub.org packages for KDE apps. Not sure what the point is here.

canonical are not the ones doing the maintenance for those apt packages. the debian team does that.

This is wrong in two ways. First, Canonical are the primary employers of a lot of Debian developers, including to do Debian maintenance or development. This includes at least one of the primary developers of apt. Canonical also upstreams a lot of their work to Debian. Second, of the three (!) whole packages that Canonical decided to make transitional packages to the snap, none were coming from upstream Debian. Firefox was being packaged by Mozilla (and Mozilla were the ones who decided to move it to the snap), Thunderbird’s package had been something Canonical was packaging themselves due to the Debian/Mozilla trademark dispute that they never moved back to syncing from Debian due to technical issues with the port, and Chromium was, at least at the time, remaining frozen at old versions in a way that was unacceptable to Ubuntu users.