It seems like their disclaimer is based on two points, this:
[1] Most non-Ubuntu distributions don’t have all the underlying security tech that snaps require (most of the time what’s missing is apparmor).
And this:
[2] but your kernel needs to be properly configured
On the first point, there are many other distros that use Apparmor by default (Most distros in the Debian/Ubuntu family), Some OpenSUSE variants, and with Arch there is no default but Apparmor is available.
On the second point, they don’t specificy but I believe they are referring to a Kernel patch that Ubuntu has been trying to get upstreamed for some time. It is available as a standalone patch for any distros that wants to implement it, but if it could be upstreamed all distros would benefit by default. There is a thread tracking progress and discussing the issue here
I believe you can use this command to check your own system setup (if you have snapd installed):
snap debug confinement
the desired output is “strict”