This is the part that isn't clear and is by far the most interesting. At what stage and what point did the GDID get correlated with a tool/web request. As is it almost sounds like Microsoft "telemetry" gathers everything and they did a bulk search for certain activity, pulling the GDID and correlating it with a user.
Good question. My understand is that it was licensing:
Hackers cloaked IP address -> VPN license -> Windows GDID -> Hacker's name.
From reading the official criminal complaint [1] it looks like Microsoft literally logs all web requests along with the GDID and sends it over as "telemetry". It basically associates the URL, the client's IP, and the GDID together.
Or I suppose it's possible that it only sends the domain and not the full URL, but that's enough for the police to go to the hoster and demand logs containing the full URL for said IP.
1. https://www.justice.gov/usao-ndil/media/1450651/dl?inline