Misdialling short numbers on a rotary phone was easy and common. If your finger slipped out of the dial it would return at that point and dial a shorter (lower) number. The most common point for a finger to slip was at the start of each turn, so if your finger slipped you would mostly dial ones or twos.

You cannot dial a 9 in this way.

The amount of time it took to dial 999 was inconsequential. Nothing was instant in the days when this protocol was invented.

There is nothing special about 9 in the rotary dial. It sits between 8 and 0. The reason for choosing nice was not because of misdials.