https://www.unrealengine.com/eula
the Unreal Engine is not Open Source. Not least because you can't share the source code without restriction.
As I understand it, the difference between free and Open Source is a difference in philosophy rather than a difference in licensing. For Free Software people the importance lies in freedom, but for Open Source people the importance is that it's a better way to develop software.
For example, a Free Software person would always use free software even if was inferior to a proprietary alternative, because they wouldn't want to give up their freedoms. An Open Source person would use whichever is superior, regardless of licensing. I'm in both camps but I tend to use the term Open Source because Free is confusing because it means both 'no charge' and 'liberty'.