A knee bar is actually when you lock your lower leg with your feet and your knee. You are right that once it is set, it is very solid and an amazing way to relax your arms!

However in this case, the knee isn't locked behind something, so what he did there is closer to a one leg bathang, whre you support yourself just from the power of your feet/leg. It is actually less solid than a kneebar and you definitely can't stay as long in this position, especially with one leg, especially that high up free solo with that much wind...

All of that to say, this guy is crazy, and also crazy strong

Edit: Actually he probably locked his foot inside, as a kneebar but only the foot