It's still updating the entire application. Snaps do perform "delta" updates - so instead of Apt downloading and unpacking a new archive (12 MB), it would just be a 239 KiB update (the size of the changed file that contains Boutique's app listing).
Kind of, but the two formats work differently.
Apt - The maintainers of a package in the Ubuntu repositories decide when to update. Many packages are not the latest throughout a release cycle, since packages need human review and testing (in proposed) before they are available to users. Many packages are inherited from Debian.
Snap - The developer decide when to release - but it's ultimately down to them to be sure they did enough testing before they push out an update. As a wild guess, most snaps are user applications then system components, so breakages shouldn't happen. I agree with the concern that the user lack precise control over how/when they update.
Snaps, in my opinion fit better for proprietary software (like 
Spotify), whereas FOSS software works great with the Apt model where it's transparent (retrieving the source and any patches) while being sure someone confirmed it works with that release.
PPAs (some official, some community) can provide the latest software too, but the system can sometimes still be outdated. For example, my VirtualBox 5.2 in 16.04 had theming issues as one of the installed dependencies were outdated (Qt). If it was a snap, in theory the self-contained feature would allow it to have all the latest dependencies independent to the rest of the system 
... while sacrificing disk space and control/knowledge when it updates. 
Another example: AptDaemon is the backend for 
Software Boutique (at the moment). It's now deprecated and we were asked to remove it. Since it's a snap, we continued to keep it - until Software Boutique 2.0 which will replace it with PackageKit.