That one's not actually true though, e.g. the lowest-altitude formation-flight record currently stands at -422m (over the dead sea).
I don't think its really possible to fly any lower. The dead sea is the lowest place on earth, and -422m is even pushing that as it must have been a very dry season when they did the flight. Unless the dead sea evaporates even lower, flying below -422m is going to be pretty hard.