Absolutely true, but that isn't what happened (at least, not yet). Most companies found the fee reasonable and worth it to have us maintain the project and not have to fork it.
You must have incredibly good analytics to know exactly what companies are using your nuget package. Did you embed a phone home?
Checking whether it's their own compile or official binaries is also simple if they use an extension like the official Util one, which many do: It embeds a binary that implements the custom action, which is signed by FireGiant.
As for turning this into analytics / statistics, I imagine you could just download every MSI from winget and just check if they contain a FireGiant-signed extension dll.
But it is easy to tell when an installation package is built by the WiX Toolset (or any other tool) when you know where to look.