It doesn't. It just means the PlayStation 4 parts are simply not open-source. MonoGame is available under the MS-PL, and MIT-alike. The PlayStation 4 backend is available under non-open-source terms to authorized PlayStation 4 developers.
You mentioned some flamebait claim, without any substance. If you can't elaborate (like provide a link to the actual source for instance), I'd assume it's a fake, and their PS implementation is not open source and you are simply trolling, like usual.
The whole point of an NDA is to prevent access to the source without authorization. I.e. incompatible with open source by definition.
