KDE locks the screen out of the box, in fact looking through the GUI options I currently see no way to do a screensaver without locking. Though granted you don’t need the compositor to do that anyways.

What’s true is that not just any program can lock the screen under wayland, it has to be the compositor or a program the compositor grants the power to do so. That’s so that “press alt-tab to login” type prompts can reliably sniff out keyloggers.