You are almost there.
> but you've got to assume that they haven't tested randomly but tested people who have been in touch with known cases
Yes. Well with symptoms etc. So of the people who got tested they are much more likely to test +ve. I agree. Which means real cases are massively more widespread than those tested.
It is believed here in the UK that this has been fairly rife for a while now, but those who got it are not statistically counted anywhere. The real fatality rate is way less than 0.9