I have long-length thighbones and flying is an experience in complete discomfort for me. However, in the case of this United flight, I think the problem was not knee pain, but the top half of the seat invading his upper torso comfort zone. My understanding is that the man in question wanted to use his laptop and that’s impossible with it shoved up under your throat.
If she did, and the passenger told her it would be uncomfortable for him because of his height and she did it anyway, it was rude of her to do so. Anyway whatever happened to courtesy?
I've had several occasions when the seat recliner in front of me nearly crushed the lid (screen) on my laptop. The lid would catch under where the top of my tray table would be. When the person leaned the seat back my laptop would be crushed into the tray table by the lid. Nothing broken but I have had to act quickly or in some cases temporarily stop the recliner to rescue the laptop.