Just guessing here, but it seems reasonable at least in my mind:
Exclusive to 1st party sounds like it really means 1st party are your beta testers. Unlock it for them, let them release a game or two, and see how things like
OS performance
Background music
Party chat
Secondary apps/web browser
Etc. all perform in the real world before opening the flood gates to all developers. In the name of stability of course
Another advantage of doing it this way is that your first party devs can work with middleware devs to start utilizing the extra core (and working out the bugs), and hopefully by the time it's available to all devs, the major middleware software already has support for it and it's an easy upgrade for third party devs.
Just think: boom, one day you suddenly have access to an extra core, and all your middleware and libraries already take advantage of it.
Anyway, I'm no insider, but this seems at least reasonable to assume... I think.