10,000 attendees (per ABC article) isn't really that many in the scheme of things for a city of our size. If my maths checks out, we'd need to have dozens of undetected cases across the city before it becomes remotely likely that someone in attendance was carrying it.
Part of the problem is that people cannot know whether or not they are carrying the virus without being tested.
Common 'flu has an attack rate of 1.3; the latest research suggests corona is between 3 and 4.