I dont tihnk its a hard and fast rule. The justification it gives is that both are usually trained form birth, and thus one cannot be both, but on the other hand, if someone who has not trained in the class from birth can later MC into it, such as their example of a fighter taking some sorceror levels before going over the the dark side, it stands to reason that one can do this regardless.
Otherwise, one could never MC into either class without having training in that vlass in their backstory, and such training would usually not logically be able to result in a character in another class. I, for one, would most certainly allow such multiclassng IMC.