Recent testing revealed that Arch Linux, Pop!_OS, and even Nobara Linux, which is maintained by a single developer, all outstripped Windows for the performance crown on Windows-native games. The testing was run at the high-end of quality settings, and Valve's Proton was used to run Windows games on Linux.
If it has kernel level access and can run arbitrary code, that’s a rootkit.
It’s absolutely valid to call these systems rootkits.