We are really more interested in mild cases, not asymptomatic cases. The evidence seems to be that most asymptomatic infections go on to show some symptoms at a later date.
They didn't really say asymptomatic or mild case, but presumably people with (strong) symptoms would have gotten a test and already be in the official statistics.
The next study that is currently being prepared will include antibody-tests, but it seems logistics and which test to use is not a trivial matter.
