Yes, UE4 is not FOSS as per GNU, it is open source in that you can see the code and there's a large community contributing fixes and additions with every new version. I think the distinction is pedantic for most users.
> 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.
Also the whole point of this story is that license terms do matter, especially if you want to sell games commercially.
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.