You're right, it's not a sufficient condition (pretty sure Red Hat offers Linux support in subscription form), you also need to have closed source software for

You're right, it's not a sufficient condition (pretty sure Red Hat offers Linux support in subscription form), you also need to have closed source software for a platform. I don't know about the situation with Unreal, but I would be very wary of using it, considering Epic has been pushing a game Store. (And Unreal's source code is merely source available ?)