I’m not surprised by his decision since he already retired the role In the Last Jedi, came back for a cameo in Rise of Skywalker. And for those two deepfake Luke episodes.
It’s a bit presumptuous though to say you won’t be in a movie that isn’t greenlit or fast-tracked and may not even get made.
I’m not convinced Luke won’t Return in Dave’s movie. Only that Mark is done and has been for a while.