Except that if it really was 'spreading uncontrolled' and giving everyone 'the worst flu', as opposed to a handful of isolated cases which became more than a handful shortly after people started to panic about it, there wouldn't still be more vastly people being hospitalised for respiratory symptoms weeks after measures were taken to halt the spread than in late February. There might not have been much community testing going on then, but hospitals were certainly keeping records of who was coming in with respiratory problems; the well-established fact that comparatively few people were is strong evidence against the hypothesis it had already become widespread by then.
You're also more likely to develop pneumonia from Influenza than from COVID-19, especially if you're young. If you presented to a doctor oblivious of COVID-19 (or SARS), they would assume it is Influenza.
Viral pneumonia is among the most common cases of death among the elderly.
Influenza cases vary significantly between years.
There is a lag of infection to death of two to three weeks.
It would have been invisible for a long time, spreading uncontrolled.