What did I mention that’s not part of the open source definition? Btw, I’m using this one, and only mentioned redistribution, which is the first one:
The license shall not restrict any party from selling or giving away the software as a component of an aggregate software distribution containing programs from several different sources. The license shall not require a royalty or other fee for such sale.
The next big part is able derivative works, which is also not allowed as part of the Unreal license AFAIK.