Originally I believed the author may be the sole author of Menuet32, and thus could relicense to whatever they wanted. However, the release notes (https://www.menuetos.net/relnotes.htm) credit many other authors with their contributions.
The project kept growing a bit, very slowly. Seems like after v1.00 the author lost most interest. So if it is still a prototype work without any commercial usage, it would be great to have it published with source code.
Not sure if it becomes "legal" to breach the GPL if original authors die though.