With snap they can release the package a single time, and it can be used across all of their releases. I think this is the main point of snap. Being able to use it across other systemd distros is just a bonus.
Flatpak is not a solution for packaging a large portion of the types of software Canonical packages with snap, such as database servers, kernels and containerisation software like lxd.
I replied to:
Flatpak is not a solution for packaging a large portion of the types of software Canonical packages with snap, such as database servers, kernels and containerisation software like lxd.