Acting like an extravert has benefits, but not for introverts

>‘Thus,’ write the researchers, ‘the main effects of the intervention were wholly positive, and no costs of extraverted behaviour were detected for the average participant.’ The advantages were to a large extent mediated by participants acting more extraverted more often – though, interestingly, not by being in more social situations: ie, by changing the quality of their social interactions, not the quantity of them.

I was under the impression the extrovert/introvert distinction was essentially one of quantity: the introversion demand less social interactions, extroversion more. This conclusion reads to me as if it said, "It is mentally healthy to consistently assert yourself" which I take to be true regardless but that also kinda calls into question this whole distinction to me.