Yes, there are often undesirable effects associated with attaching one output to another. It's not advisable practice, but INPUTs to outputs, in this case WEAK pullups, if available, will cause no damaging effects. The example shown has external pullups on the columns, so pulling down a row should cause no trouble. The columns are presumably either open-drain, if possible, or weak pullups, else the external pullups would not have been necessary. They'd have been series resistors instead.

No indication was given, IIRC, as to which variant was in play. As a result we really don't know what port configuration options were available. The arrangement shown would probably work best with weak pullup or open-drain outputs. With push-pull, I'd be tempted to use a series resistor at the column port bits.

RE