Funny you should mention this. It's an error from 2014, where their AC is Armor Class 18 (chain mail, shield). And it too has a longbow and even a notation on using it's longsword two-handed. Yet it doesn't offer an alternative AC either. In fact, no monster in the MM has an alternative AC (except maybe lycanthropes?). It's one of the many simplifications made. And I'm fine with that as long as I never have to see "AC 15 (+1 Dex, +3 studded leather, +1 light shield), touch 11, flat-footed 14" again.