Since though quiet in
He is friendly, though quiet.
is non-essential to the whole meaning, it can be described as an adjunct. But what type of adjunct? My guess is that it is an adjectival one, but I am not completely sure because of the role that though plays.
If we consider though quiet to be an adjectival adjunct, then the 'core' of this adjunct must be quiet. Thus though is optional in the sentence, but then it becomes meaningless if we remove it.
Is though quiet an adjectival adjunct? If so, what role does though play in it?