I agree that extension does not come from pushing, but rather the hands are pulled to extension by the bat.
Good.
However, a quick check of MLB video (Albert, Mig, Buster), extension of the rear arm is underway going into contact. I'm having trouble with the idea that zero extension of the rear arm happens until after contact. Do you have any video of HL swings where there is no extension prior to contact?
If I said something that indicated I believed that then my bad. It's a case of function over form. Connection/connecting the bat to the rotating torso efficiently can be achieved with varying degrees of flexion of the back arm and varying degrees of distance between the lead arm and the chest.