The claim that snaps are a Canonical NIH thing is falsified by those two facts. Even if Canonical said “okay, we’ll distribute desktop apps with Flatpak,” that wouldn’t affect the vast majority of their ongoing effort for snaps, which are related to things that Flatpak simply doesn’t do. Instead, they’d have the separate work of making the moving target of flatpaks work with their snap-based systems such as Ubuntu Core while still having to fully maintain that snap based ecosystem for the enterprise customers who use it for things that Flatpak simply doesn’t do.
That link includes a whole lot of old things as well as blog posts about how they sped up the performance of the Firefox snap, after which there doesn’t seem to be much, if any, further evidence of the snap being slow.