Platform is whatever you mean when you say it. When Sun said it, they meant a sandbox that is all our code needs to know about. So you can go full "Dead Parrot Sketch" and say "No it isn't!" if you want.
(Sorry for that culturally narrow joke. I don't have access to YouTube here so can't give a link. If you don't know it, look for Dead Parrot. It's worth the trip.)
A good question is never answered. It is not a bolt to be tightened into place but a seed to be planted and to bear more seed toward the hope of greening the landscape of the idea. John Ciardi