"Bows require the wearer be unarmored in the base rules."
I think this is a deliberate decision. In normal D&D an archer will be lightly armoured. They aren't rocking up to the battlefield holding a shield (Like the "Light armour") or wearing full plate (Like the "Heavy Armour").
An Archer in Mausritter would be wearing normal protection like this. It's just that normal armour is taken for granted by the rules. The game assumes that every mouse would be wearing normal armour, so it doesn't give you any special advantage or bonus defense.
So your archer isn't actually "Unarmoured", they're wearing normal light protection that is not modeled by the rules.