IMHO a critical comes through the minimal opening of the armour, which is so small of a chance that you don´t bother.
For me a criitical is like wearing a (late middle ages) full plate armour and the critical manages to to hit you through the eye slit of the helmet, or the joints.
Which is also the case if the armour is additinally enhanced by magic.
But if the magic doen´t enhance the armour but IS the armour, and covers your body entirely, then a critical can´t fint the "small opening, which can be ignored, because the chance to hit it is to small to care about" isn´t there.
But the rules don´t support my interpretation.