I just asked Plako that, the reason it wasn't fully followed was that Plako was attacking one side of Whosits' empire, and Slaze the other, so they were never attacking "together". This meant that in the interests of keeping the game moving, the rule was only followed to the letter when they both attacked one of Whosits' last cities at the same time.
In other words, the rule has always been followed when it would have an effect on the gameplay.
In other words, the rule has always been followed when it would have an effect on the gameplay.