I guess it depends on your definition of fallen? I personally think the “Anakin died and became Vader” strict dichotomy is pretty silly.
It also depends whether we’re talking about before Luke entered the hut or after Luke searched Ben’s feelings.
So how do you define “fallen”? I don’t think having bad thoughts counts as fallen. But maybe that’s just me 😉
Luke doesn’t know Ben’s thoughts, he knows his feelings and his future, which I think are more telling.