Or the author clean room reversed engineered the bootloader. Or there is enough information out there to forgo the need for any internal knowledge of Windows code.
The PPC code base was never explicitly targeted for Macs, but other systems from IBM/Motorola; but because it is a 'common' platform, the binaries themselves on the NT4 ISO do not need modification.
And yes, you can find the source code on GitHub. In multiple repos!
[0] https://www.neowin.net/news/exclusive-windows-2000--windows-...