Okay, so let's be clear on some things:

The scene where Luke's hand is repaired isn't Mark Hamill. It is, however, Luke Skywalker, and it is, despite not being Mark Hamill, part of Star Wars. I mean, obviously.

John Williams wrote music for a scene in which Luke builds his lightsabre. Composers write music for things that have been filmed, not things that haven't been filmed. It's reasonable to believe that such a scene was, at some point filmed. I believe Anthony Daniels even said it was filmed, right?

Thus, even if that's not Mark Hamill, that is the scene, and it's not "fake". The corridor Vader is walking down is seen to be outside the Death Star Hangar in the film "Return of the Ewok", which makes this scene fit incredibly well between Vader's arrival and the Droids going to Jabba's. Even if that's not Mark Hamill and he's lying in that clip, it's still a real Star Wars cut scene.

That being said, people's memories are imperfect. Mark could have filmed the scene and forgotten about it until he saw it again. Things like that can jog your memory, you know.