They call it "a software tool that modifies Windows" but the tools that do the actual modifying are Win10-Initial-Setup-Script and O&O ShutUp10. Judging from a video tutorial linked in the article this is a customized ISO.

It's an interesting question, what if your tool removes the DRM from the ISO or turns Windows 10 into Windows Server.

DCMA specifically covers the removal of DRM, etc. But a tool to remove or modify anything else would be perfectly legal.