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.
Whether feelings or thoughts (the distinction isn’t terribly important as far as I’m concerned*), perceiving the future is another matter. As Yoda said, “Difficult to see. Always in motion is the future.”
And if Luke perceives that Ben might in the future fall, that is still different from perceiving that he is fallen.
*(For present purposes anyway.)