This brute force approach works much better than I expected as long as you have enough probes and a bit of luck.
But of course there are much better and smarter approaches to this, no doubt!
You mention the quality several times in the article but it's not clear how this is verified. Do you have a set of known-location-ip-addresses around the world (apart from your home)? Or are we just assuming that latency is a good indicator?
I tested against them, as well as other infrastructure I control that is not part of the network, and compared to the ipinfo results as well
However, ipinfo still appears to rely on active probing to triangulate geolocation data, which suggests they believe these routing asymmetries can be modeled or averaged out in practice.
The telco DSL and fiber in my metro area all runs through a single location where the PPPoE (hiss) concentrator is and the first hop latency from DSL interleaving swamps the latency from distance. You can someone is in the metro area, but not the county or city.
Cable company customers are a little more locatable, probably get the county.
Tier 2 ISPs have the right incentives to peer with every network in every peering point, so you are likely to get a correct result from a different ISP. Tier 1 ISPs avoid peering, so if your probe or target is on Deutsche Telekom or Cogent, your packet takes a circuitous route, passing only through networks that paid the extortion money.
Friends don't let friends get internet service from Deutsche Telekom or Cogent.
so theres funky overlap wherein on one isp you appear closer to city A, and on isp 2 closer to city B, but its same physical address.
Continental classification I'd think would be good as they appear to be coalesced endpoints, separated by vast oceans.