It was a GK error, no doubt about it. The only reason his knee came into the equation is that he initially didn't catch the cross cleanly and it dropped onto his knee. 98 times out of 100 he makes that catch as it's a pretty routine one. Was at a good height, he didn't have to move much for it and while it was hit with a decent amount of pace, it wasn't hit incredibly hard.
Alisson is boss and I love him and we are lucky to have him, but trying to do mental gymnastics to try and absolve him of the error makes you look like a nutter. Call it what it is, move on.