It's not a rule at all; what was said is none of the 13 (Well, 12) were designed with other characters in mind (i.e., there's no planned alternate heads or details intended to be other figures.) There's nothing stopping them from doing it in the future in the slightest. However, while that doesn't preclude Unicron Soldier being a Leige Maximo retool, it doesn't really line up for me.If that ever was a rule, it was a self imposed rule that didn't actually matter. It may have been a point of pride to say these Primes are special and will never get repaints/remolds but the executives really don't care about that sort of thing.
Firstly it'd be weird for them to not have any alternate plans for the mold, only to have a retool show up in the immediate next wave; additionally, have we ever see a retool of an existing leader show up in the wave right after it?
EDIT: Wait, Galaxy Upgrade Optimus Prime was the wave following Siege Ultra Magnus, so there's that one at least.