Snaps use squashfs, so instead of a hardlink farm, you have self contained fs images.
> Snap is similar, although it requires AppArmor, so can't run on many popular distros,
I'm not sure why you continue to repeat this factually incorrect information. AppArmor is not required. In fact, guessing you being a Fedora user, you can verify that by installing snapd from your distro repositories.
However AppArmor is used for providing confinement of snaps (aside from sandboxing which requires the same interfaces in the kernel as flatpak or docker do).
> and I think there's only one runtime that you're required to use, so no "mix and match".
Currently there are runtimes based on Ubuntu 16.04, 18.04 and upcoming 20.04. There's some work to support freedesktop runtimes, but it got stuck on defining metadata for describing security profiles of apps using given runtime.
> FlatPak SDKs provide this.
> Snap has an SDK as well, although note the singular, Snap is exclusively controlled by one company.
Wouldn't call it an SDK, but there's a tool called snapcraft to build snaps in a convenient manner. You could just as well use 'snap pack' or mksquashfs directly.