My old (not old, old but recently sold) E90 had the same dimming setup. Yes the exterior mirrors would dim, but only to the extent of high beams not dazzling me from behind. Maybe they did not even dim, perhaps they had an anti dazzle coating on them. Or both.
Same setup in the F30.
328i M-Sport, Estoril Blue, Carbon XP 35 Tint, front heated seats, black Dakota, Sports Suspension, Anthracite Headliner, HUD, Pro Nav, Sunroof, hk 600w, Bi-Xenon, 19" 403M wheels, Reversing Camera, DAB+, Xenon, BT Ext Connect.