Basically you can get normal Windows already without paying from Microsoft itself. But on this site they also publish Windows LTSC ISOs which you can not get directly from Microsoft unless you are an Enterprise.
Plus, the scripts allow you to activate all of these Windows version and I think also Microsoft Office.
The script is abusing bugs that are long known but not fixed. And Microsoft does not seem to care, as the script is hosted on GitHub (which is owned by Microsoft) since a long time.
The repository also has ~60k stars, so it isn't "secret" that this exists.
