What he is talking about is the shift boot and leather on the knob is one continual piece.
So to go with an aftermarket knob, means you have to cut the leather at the base of the OEM knob to remove it. (You are going to want to remove the entire unit first though; knob and boot) Then you have to use a zip tie to hold the leather in place on the shift arm since it will not be attached to the knob anymore.
Does this make sense?
This thread should help: