Ubuntu, Mint, and Debian all still rely on the Linux kernel, though. A better comparison might be how LineageOS is technically a fork of the dead CyanogenMod project, but even before the latter fully imploded was different enough to be its own thing.

Both of them still relied on the existence of AOSP though, for new features, bug fixes, hardware support, certain core functionality, etc. AOSP is a lot bigger than just the Linux kernel, and because of the tighter coupling between hardware and software on mobile devices, there’s a whole other discussion about creating a real non-Android OS for them, but I think that’s a closer parallel.