I would have to say that a consistent release point is only one factor in pitching command. Consistency in release is not any more important than the other primary aspects of a pitcher's mechanics.
Grips are important to movement, but so is finger pressure.
And I wouldn't recommend taking too literally the "absolute outside corner at the knees, 6 out of 10 times". Yes, command of the fastball is the first objective. Followed by command of a changeup. But if someone is talking about absolutley not moving the mitt at all, and having the pitcher hit it six out of ten, most pros can't accomplish that (at least in a game). {And before anyone argues the latter statement, I recommend they watch some pro games. Mark down for each pitch whether or not the mitt moved. The results will surprise some folks.}