I don't think this is necessarily the case, in either the positive nor the negative version, FC.
If I decide NGA has some deficiencies in ground work (and it does, as commonly taught), that doesn't impair my ability on the ground. It just means I need to figure out what to do about it. I can just study BJJ separately, and integrate them in my own usage, and that solves my problem. But not my students'. So, my alternative is to improve my abilities on the ground and integrate some solid new principles and practices into the art as I teach it. Perhaps this integration is what you're talking about. If I decide NGA is inherently flawed in its ground work (as in, the art cannot contain good ground work), then I have a problem. But if I just decide the art is lacking in its current form, that doesn't limit me.
On the other hand, if I decide that NGA is quite good in its ground work, as I learned it (and it wasn't very good), that does not make me better at ground work. Just because I decide the art is good on the ground, that doesn't make me more capable on the ground. But once again, maybe I'm missing your actual point. If you're saying that I could decide there's capacity in the art for good ground work, then I have room to figure out what parts of the art translate to effective ground work.
I guess my point is that there's a difference between my belief of what the art's current state is (that's not a "belief thing", as DB put it), and my belief of what the art's capacity is (here, belief can drive response).