.. which allow you to configure the outputs as Push-Pull and the inputs as either open collector or weak pull-up.

You can do it either way, if you use OC inputs and active high outputs or active low outputs and pull-up inputs.

for a non-LPC it still works with active low outputs (pull-ups take care of the other) and pull-up inputs.

If you have a longish cable from the keypad to the uC you will want to reinforce the pullup, but if keypad and uC are in close proximity, I see no reason for that.

BTW I do NOT use the key interrupt, it does not make debouncing any easier than a straight timing loop, BUT in a battery operated app it is valuable as a wake up interrupt.

Erik