From the link I quoted above, cmp's post mentions this:
...
if (weapon_type == 'two_handed' or weapon_type == 'polearm') and (has_shield or mounted):
raw_damage *= 0.85
if weapon_type == 'polearm':
raw_damage *= 0.85
...
So yes, it seems like if you either have a shield or are mounted (or both, as "or" is almost always inclusive), you get the 15% dmg reduction (multiplying by 0.85) twice, which gives the 27.75% penalty (or you do 72.25% the raw damage).