I'd like to hear your explanation how an IQ of above 200 is possible and what that would actually mean.

It means that the mean and standard distribution have been calibrated to a population, but that the population kurtosis is significantly non-normal

Its only possible if there are about 10x more humans.

Incorrect. It's also possible if human intelligence isn't normally distributed.

With a population of around 80 billion, the smartest one person would have a z score of roughly 6.6 and an IQ of roughly 200. This is calculated from a mean of 100 and a standard deviation of 15, which is how it's defined.

Only if intelligence of the human population is normally distributed.

So, as I've been saying, you just put everyone's test scores in order from worst to best,

No you don't. You have invented this unnecessary step.

calculate the z score of the person you're interested in, multiply by the SD (15) and add the mean.

No, because the "person" and the z score have no link.

It is also the case that for populations over 80 billion, you can have negative IQ scores, using the same logic that was used for a person with an IQ of >=200.

If a rock has zero intelligence, how can something score lower? Negative intelligence is impossible.