It's not based on Ubuntu Snaps. It's a custom development. It predated Ubuntu Snaps by I think 2 years? I don't see any obvious reason to switch to Snaps now, since it's all been really reliable so far. The downloads are done using zsync. Usually a system update is less than 5MB total.
Otherwise this is really good to know. I guess I don't run RPi's enough because my lazy unplugs haven't been an issue...yet.