Only recently. Hmmm....The excavations in Iasos started by an Italian team in the head of Prof. Dr. Doro Levi in 1960, and continued by Dr. Fede Berti until today.
It is merely a very poorly written article. The tunnels and sewer may have been in place much earlier and covered with ash. But the newer ruins weren't.
I’m perfectly happy if the ash is A) actually volcanic in origin, instead of being the result of destruction of the town by invaders, and B) from Thera, but I’d venture to guess that this hasn’t actually been scrutinized by appropriate specialists. A classical Greek city (founded in the 6th or 5th c BC) buried by Theran ash in 200 BC would be about right. Probably not reality though.