Is it really pirated? As far as I know they have an agreement with stuntzi or other MOS Team MUI devs.

To measure the incompatibility is really difficult though. It could help if there was an all in one program like the original demo programs using all functions from MUI5. If the same compiles on OS3/MUI4 and works perfectly then they have done their job. If not you should see what is incompatible. This would also be useful for Zune on AROS. But the question is why any of the sides should code this program. The OS3/4 team knows that it is behind. The AROS team knows that it is far behind. And the MOS team should develop their own software and improve MUI instead of doing the work of others.

In general if a dev is targeting MUI4/OS3 or OS4 then he does it wrong from the start and should be slapped for it. If he wants to do it right then he should develop it for the newst version MUI5 and see what does not work with MUI4/OS3/OS4. Then tell this forked MUI team what is working wrong so they can repair it.