Processes that run on the same system can run as different users (including kernel) which is used for privilege separation. This can still allow a program in userland to peer into otherwise restricted system processes or the kernel. Every system is a “multi-user” system, even if there is only a single human user.
I always kind of thought of KDE as “okayish” but ever since plasma 6 I’m a convert! Hope to see some excitement for these projects in the coming years - it feels to me that KDE is reaching a maturity that could attract a critical mass.