MAD isn't doctrine so much as an observation & logical position.

`If enemy has nukes, and you nuke them, they will send all their nukes to you, you send all your nukes to them. Everyone dies.

It's a description of the status quo between nuclear-capable nations. It's a guiding principle, based on game theory & human psychology.

But it's certainly not a natural law of the universe, and whether it's anybody's doctrine is irrelevant—it's still the chain of events that is likely to occur in a nuclear conflict between two or more nuclear powers.

So, 'be mindful of MAD & don't be stupid'. I think that's the primary function of MAD as a concept. It's like a warning sign, or cautionary tale.