Ground work is just what you do when you're on the ground. Being able to deal with a grappler isn't the same as having groundwork. It may prevent some groundwork, but grappling takedowns aren't the only way to end up on the ground. NGA has a few tools for getting back up, but the traditional components aren't strong, so it's a weakness in the art...unless the art is trained to fill that gap.That's one way of seeing it.
It could also mean that the method already has the tools to deal with a grappler, without becoming a grappler yourself.
I realize your method is a form of aikido, so it already is a grappling variant, in lose terms. But I don't think you need to play someone else's game in order to beat them. Play your own game, and beat them.
So, the ground game isn't about beating a ground-grappler on the ground. My chances of staying off the ground are probably better than my chances of beating them while down there. But if I end up on the ground against anyone (off-balanced by a kick, for instance), tools to work from the ground until I get back up are useful. Now, I might decide I don't need them, but that doesn't change the fact that NGA has a weakness there - I'd have just decided it's not a significant weakness. A good example of what I consider a non-significant weakness is in high kicks. We don't train them much, as a whole. I also don't really see it as a problem, because I have enough tools that I don't really miss the high kicks I don't have. It's a weakness that doesn't bother me.