MonaOS: another AtheOS/BeOS fork/clone ?

Code-wise it doesn't resemble AtheOS or BeOS. I presume the copied interface elements can be seen as a "homage" or just as what was to hand. It's largely the work of a single person.

Actually the other screenshots show window decor that more closely resembles Classic MacOS. So don't deduce too much from screenshots alone. Haiku too can change parts of its appearance to match a few other OSs (the setdecor command). There are also BeOS skins for Linux.

An OS in which executables need to be suffixed with .exe to be picked up as such by the system is unlikely to have BeOS anywhere in its heritage.