This is blazingly, dangerously incorrect. First of all, cases are undercounted because China explicitly does NOT COUNT asymptomatic cases. That’s why they could proclaim “no new cases” even though they still find people carrying the virus today. Second, your “not entirely true” assumption that everyone was exposed to the virus is absurd. If everyone had been exposed, the Wuhan lockdown would have had little effect — in fact, there would have been no need for a lockdown, because almost everyone who could get sick would already be sick.

If you want a hand-waving way to estimate the infection rate, epidemiologists have already done if for you with the R0 number. And, unsurprisingly, it paints a very different story.