The article makes it sound as if UE4 was closed source or at least unwilling to take pull requests, which is not the case. Also, as far as I can tell from skimming, the UE4 source is really comprehensible and you can fiddle around with it, add custom shaders and objects and learn from it.
Though I do agree that an absolute beginner should start with a less powerful engine. I learned a lot tinkering with Cocos2D-X that helped me grasp basic concepts of OpenGL.
Unfortunate if I came off that way, that was not my intention. Just tried to keep things short by bundling all the 3 mentioned engines into "different brands of free".
UE4 is indeed open source, though not open to the extent of your average open source project, YET. But I'm not at all trying to give them a hard time about it. I know these things have to happen gradually. What they're doing is great.
I think smaller frameworks like jME, LibGDX, Cocos-* will still live quite a while, exactly because of the community and contributors momentum the bigger engines lack.
BTW, I wanted to have a look at the jME source and noticed that the website misses a GH link, you should definitely put it up there.
Whose definition?
Also, I never said it was open source, but merely that it's not closed source which is true, since it's somewhere between OSS and CSS.
A quick wiki visit says the term open source existed before the OSI. But that's not really important.
I just wanted to point out that UE4 does a better job than many other engine (and software!) vendors, since they give you a peek at the source and I think this should be praised, rather than scolded for not being totally free.
Though I do agree that an absolute beginner should start with a less powerful engine. I learned a lot tinkering with Cocos2D-X that helped me grasp basic concepts of OpenGL.