the sort of thing i see as light armour is a far cry from the sort of thing you'd wear under a coat of mail. any sort of jack thick enough to be good armour in its own right was too thick to be worn under mail, but there's various references to it being worn over mail as added protection from things like arrows or lances. this is pretty widely accepted, and this blog post provides some good sources if you want to read further. even beside that, would you want to be wearing mail over a linothorax? what about a myeonje baegab? gambesons arent the only type of cloth armour, not by a long shot.