That's correct. NG+ doesn't stack, meaning you can only start one NG+ playthrough. If you want to play NG+ again, you have to load up the vanilla save file again.
So, you're saying that I will never be able to play TW3 starting out at the level cap with optimal skills and gear? That is so, incredibly, disappointing. That being the case, in a way, this makes NG+ currently pointless. If I play through NG+ now, then I'd be missing a great deal of items/money/experience that I would have gotten from the expansion content on my first run through the game. I would have to abandon my NG+ file (may as well delete it at that point), go back through on the original run of the game, play through the expansion content, and *then* I'd be able to start NG+ with everything I should have had in the first place. That's a lot of wasted time and effort, doing it that way.
The primary point/goal of NG+ (in its purest form) is to allow players to carry over basically everything that isn't a story related item (to hell with gathering gwent cards, only to have to do it
again), without limitations on how often it can be done with a single save file. Dead Space 2 and Mass Effect 3 did it best, among modern video games. But, those games don't have potentially great variation in stats between the player character and enemies, so it required less work to balance out NG+.
But, you guys already worked out the balancing for TW3's NG+. So if the work has already been done to balance it out, why stop us from doing NG+ in perpetuity? It would have been far better had NG+ not been free DLC. Instead, it should have been payed-for DLC that is a true, full-fledged NG+. I feel so strongly about this that I think this is borderline fact, rather than just opinion. There is no good reason (that I can see) to hold us back like this. Please, give us true NG+, and not NG+ Lite.