I believe that's the case. I kinda remember hearing that the PS3 version might have been in development. I think the rumor was that it might still be.
I mean, it's kinda crazy when you consider that the first Witcher game was in development, and supposedly near completion (I believe it was also the Enhanced Edition, but titled The Witcher: Rise of the White Wolf) before that got cancelled because of distribution rights or something.
My understanding is that Rise of the White Wolf was not a port, and that this version of Assassins of Kings isn't either. And you're right, the Playstation 3 is apparently more complicated to build a game for, so they could just be on a longer dev cycle for it.
Or maybe they just didn't have the resources to work on this, the PC version and the PS3. They don't seem to be that large a team, so it could probably be just as simple as that.