Unreal is “source available”, not Open Source. There’s a big difference. With any Open Source project you can legally fork the project, distribute your custom version of the code, create a community around your variant… “source available” has none of that. The Unreal EULA is more permissive than most game engine licenses (with the obvious exception of Godot) but it still comes with plenty of restrictions. For example:

You are permitted to post snippets of Engine Code, up to 30 lines of code in length, online in public forums for the sole purpose of discussing the content of the snippet or Distribute such snippets in connection with supporting patches and plug-ins for the Licensed Technology, so long as it is not for the purpose of enabling third parties without a license to the Engine Code to use or modify any Engine Code or to aggregate, recombine, or reconstruct any larger portion of the Engine Code.

Which pretty clearly does not satisfy the Open Source Definition.