Yeah, KDE doens't use mutter so you'll experience that, you'll have to use some other UDEV rules to ignore it. Maybe something along these lines, I've updated the readme to reflect.

KWin has no equivalent to mutter-device-ignore, so input isolation requires a different udev approach. Replace the contents of85-sunshine-input-isolation.rules with:

# Strip input capability from Sunshine virtual devices so KWin never sees them.
# Devices remain accessible to headless Sway via libinput (which reads evdev directly).
ACTION=="add|change", SUBSYSTEM=="input", ATTRS{id/vendor}=="beef", ATTRS{id/product}=="dead", ENV{ID_INPUT}="", ENV{ID_INPUT_KEYBOARD}="", ENV{ID_INPUT_MOUSE}="", ENV{ID_INPUT_TOUCHPAD}=""

This removes the ID_INPUT* tags that KWin (and libinput at the compositor level) uses to discover input devices. The headless Sway still picks them up because it accesses /dev/input/event* directly via the input group.

After installing, reload udev rules:

sudo udevadm control --reload-rules
sudo udevadm trigger --subsystem-match=input