For the last few weeks there has been widespread testing of asymptomatic people. E.g. teams showing up at a supermarket and testing everyone, testing everyone associated with a school that had a pretty large cluster.

It is true that still most tests are on symptomatic people. But if there was undetected community transmission then you would expect some of those people to develop symptoms and eventually get tested and test positive. (Currently the PM is saying "if you have a sore throat or a sniffle, get tested" --- the bar for symptoms is extremely low.) So it's reasonable to assume that there isn't significant undetected community transmission.