It's worth noting that Ubuntu support and Snap support are orthogonal. The Flutter work here generates standard ELF binaries that can be distributed outside of Snap, and Canonical's contributions to create a high-quality desktop embedder for Linux are applicable across all distros.
I'm still excited because when Canonical backtracks on Snap in a few years (like they seem to always do when trying to push some homegrown aspect of Linux), their Flutter work will still be there and will have seen years of improvements.
That seems like it would be great, except for that it would still be Canonical's work and with everything they do it would probably be better for everyone if they did not bother.
That's not true, their upstream work for Gnome shell after ditching Unity is very appreciated.
Their announcement also pointedly remarks that the Flutter work will benefit "most distributions" of Linux. That's another good sign that this work won't be too tied up in Canonical's fiefdom.