I don't think you can depend on Covid test results, especially if the samples were taken in the early stages of the disease.
False Negative Rate of Covid Testing
Even where the test itself is reasonably accurate there can be issues with timing, collection, handling and of course, just general bungling.
Australia's population, beyond a few small clusters, has not been exposed to Covid until the past few weeks.
We had numerous documented false negatives issued recently and I know of someone locally who came home really sick and tested positive, then some days later, when the partner developed symptoms and was tested, it came back negative which is highly unlikely under the circumstances.
Another point to remember is that Omicron did not wave it's super-charged glycoproteins and make Delta disappear.
In this population Delta is alive and kicking and just getting started.
Delta is likely to do more harm in the long run than Omicron, because of the differences Sierra referred to above.