I'd say you don't have to drive the thing push-pull. The schematic to which the original reference was made contains external pullups on the columns (?) so they'll be pulled high unless a key is pressed. True enough, you have to consider the output drive of the row pins if the columns are driven high. However, that's typically safe, as most 805x types can sink considerably more than they can source. The maximal interval associated with the short-circuit current is generally longer than what you'd use to debounce. In cases where that doesn't apply, one should probably have series resistors on the driven rows, but, again, be careful, as you then have a voltage divider and may not reach your MCU's Vil max.

RE