$SUBJ can't possibly work, other distributions (like openSUSE) use it as well. There should also be a way to override the command for regenerating the initrd using a CMake variable.
Yeah that detection is a bit poor. I guess OpenSUSE does not use update-alternatives for setting a default.plymouth. As I had a nother issue on ubuntu/debian with update-alternatives I added this patch that checks with update-alternatives --list default.plymouth if the plymouth theme is already registered with alternatives system or not. I think this could be used (if it fails on opensuse as it should if there is no default.plymouth) to skip the rest of the stuff. https://phabricator.kde.org/D5942
That patch landed years ago; marking as fixed.