I’m not familiar with what Suse is doing, but that doc seems to imply that there is a fundamental difference in the configuration between Gentoo and Suse.
I wonder if there is something in the Suse docs that describes how they get a rollback option into the grub menu.
I agree that the Gentoo wiki pages around btrfs and snapper seem a bit lacking.
It annoys me that you aren’t supposed to close input tags. At least browsers tolerate self closing them even if it is out of spec.