If dsp/exchanges just required 1k or 500 users be in a retargeting pool (or list of device ids) then this problem would be solved.
As for knowing how many users use a specific app in a location, that is an extremely fuzzy number and I doubt the accuracy of it. Almost no exchanges show you how many auctions you lost, so just finding out how many uniques you served to is flawed and much smaller than the real number.
> "This is so easy and it's industry-wide," says Tadayoshi Kohno
Maybe across the spying/intelligence industry, but advertisers don't care about individuals at all. This is an interesting experiment, but most platforms don't enable this type of tracking and no advertiser would ever need/want to do it.