IMHO, as far as the general public is concerned, it's entirely a naming issue.

Copyright issues is down to the involved parties to sort out, and if they don't care to protect their IP or if they decide to share sources or whatever, it's entirely their call.

But naming confusion concerns people in the "user space" (like developers in this case). Naming this "MUI5" is like releasing MorphOS or AROS using the "AmigaOS4" mark (registration aside) in parallel to Hyperion's published OS. Although many similarities in API, there are differences that makes them *not* directly interchangeable, even incompatible. It's not the same thing. So using the same name is indeed confusing, and a rather dicky thing to do, when done so deliberately as in this case. Same when MorphOS devs shared SFS sources to open up for an updated Workbench 3.x port, and off they ran to create an incompatible OS4 port, released using the same name. There are many documented cases of confusion (and possibly lost data?) because of this.

And maybe you should meditate a bit over jacadcaps statement above (which is only a confirmation to several hints by other MorphOS developers, in this thread and others): "The fun part is that the '5' isn't about MUI itself at all."

Maybe the joke will be on Böckelmann & Maus expense in the end...?