It's possible to design a ribbon bar that does not infringe on MS patents. IRC there are only valid patents for specific parts of the MS ribbon bar, not the whole bar.
"Although Microsoft popularized the term with a new meaning, similar tabbed layouts of controls had existed in previous software from other vendors, including 3D Studio Max R3 and later, Adobe Dreamweaver, Borland Delphi, HotDog and Macromedia HomeSite." https://en.m.wikipedia.org/wiki/Ribbon_(computing)
Yes, it's closed source. It's only free as in beer. Their website says: "What's the catch? There is none." It seems they offer this as a free product because they have a paid one with more features.
https://www.freeoffice.com/en/