qdbus org.freedesktop.ScreenSaver /org/freedesktop/ScreenSaver org.freedesktop.ScreenSaver.SetActive false
does not unlock the screen.
This seems to be a deliberate decision (https://invent.kde.org/plasma/kscreenlocker/-/blob/master/interface.cpp#L117).
It is however possible to unlock a session via logind, which makes this restriction rather pointless.
It's not pointless. logind is a privileged process on the system bus, so we know it's coming from there. ScreenSaver on the other hand is a user service that anyone could just claim and tell us to unlock.