[tremon]

the simplest explanation is China didn't figure out what was happening for a while

It's even simpler than that: there was no need to figure anything out. If a few people present with flu-like symptoms but appear to suffer above average from it, Occam's conclusion would be that those people may have had an underlying condition that exacerbated the severity of their flu. There is zero need to go looking for zebra's until you identify a cluster of similar cases that all have above-average severity.