Curry was a superstar, Klay was a high level All star that complimented the team perfectly. Dray is an All Star that also fit his role perfectly. On top of that you had a 6 man in Iggy who was a former All-Star himself and they had one of the best benches in the league. If you think they overachieved do you think the Pierce/KG/Allen Celtics overachieved as well? They were essentially a younger, in their prime and better shooting version of that team.
EDIT: And Harrison Barnes who isn't the All-Star we'd thought but as a 3rd/4th option fits well into that role. Currently averaging 18-19 on 47% shooting and 35% from 3.