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.
But that’s not what Luke perceives. What he sees is a future where Ben does terrible things which leads his to a momentary lapse of judgement.
There’s no question of “Ben might fall” Luke makes it explicit that Ben had either already fallen or was deep in the process.
What Luke sensed in Ben’s feelings:
“I saw darkness. I sensed it building in him. I’d seen it in moments during his training. But then I looked inside, and it was beyond what I ever imagined. Snoke had already turned his heart.”
What Luke saw in Ben’s future:
“He would bring destruction, pain, death, and the end of everything I love because of what he will become.”
Saying Luke merely sensed Ben’s “bad thoughts” is dramatically underselling the situation.