And why should the fall of his nephew not shake him to the core?
It might. But that’s not what happened. On the night in question Luke sensed great evil and ultimately saw a scared boy, not someone who was fallen.
Or both, but let’s just ignore half of what the movie told us!
Again: what happened in that scene did not depict that Kylo had fallen. I’m not sure what in your mind counts as having already fallen.