Pozzovivo has always been this good. It's just he weighs about 50kg soaking wet and has been susceptible to illness and injury and terrible at placing himself in the péloton as a result. In 2010 he was great in the warmup races (winning on Pampeago in Trentino), but was battered and crashed in the crosswinds on stage 3, got ill in the cold on stage 8 and retired on stage 13. Last year he got involved in a crash early on, and soldiered on until Zoncolán before dropping out.
This year the Giro has been comparatively warm and the péloton comparatively docile in the early, boring flat stages, which has enabled him to arrive at his playgrounds of the mountains in better condition than usual.
He hasn't been picked up by other teams because he wants to ride for Colnago; he has been approached by other teams at times but chooses to stay where he is. Plus, his general weakness and fragility would mean he would likely be a helper elsewhere, whereas where he is he is a leader.
Maybe he's doping, maybe he isn't. I wouldn't like to say. But it's not like he's suddenly changed as a rider, like he's suddenly found a new product that turns him into a mountain goat par excellence; he's always been one. So he could have been doping all along, or he could have been clean(ish) all along.
It's not too dissimilar to Mosquera really - if you pay attention to the national scene you'd be aware that it isn't the surprise it seems to the outside world to see him performing like this (plus Pozzovivo is a few years younger). It could have been great talent obscured through years of domestique work in the Portuguese péloton and being on the wrong teams at the wrong time, or it could have been EPO. We couldn't know for sure until he actually tested positive.