Important to note, neither is Unreal really. It's not a pedantic distinction here either, as it affects how far you can take a project that modifies the core engine.
Not that I'm against Unreal in any way, it's just important to keep in mind.
Also the whole point of this story is that license terms do matter, especially if you want to sell games commercially.
> I think the distinction is pedantic for most users.
In a technical discussion, there's no reason not to use the proper terms for things.
The distinction is that the code you write belongs to Epic. You become an unpaid contractor.
https://blogs.unity3d.com/2018/03/26/releasing-the-unity-c-s...
UE4 is open to every last bit.
The reason it's not open source is the royalty clause, not lack of ability to see/edit/modify/fork the code.