They might simply like Mint’s Cinnamon over Ubuntu’s GNOME. That’s a valid choice.
Cinnamon with Wayland is still in testing. X11/X.Org is unmaintained software and is less secure than Wayland. GNOME is the only desktop at the moment that actually protects the screen from arbitrary recording by applications. Just food for thought.
Plasma supports wayland as well. On distros where it doesn’t ship by default all you have to do is install a package.
Yes, but Plasma doesnt protect against screen recording. The Devs expressed interest in protecting against arbitrary screen capture, still work in progress. See this issue: https://invent.kde.org/plasma/xdg-desktop-portal-kde/-/issues/7