Like I said before, that is a statement only someone with tiny little feet would continue to repeat.
To the OP, if you get a chance check out the Nitro Blacklight they offer it in camber and GW. Its a mid wide so the waist width should be ok for your size 12's.
As soemone with very tiny feet (7.5 boots, hey my wife is happy
) waist width isn't irrelevant, but isn't determinative by itself either.
It's a guideline though. That tells you in general if a board is worth looking at, or completely out of the question.
I rode a Coda which was way too wide for my feets but it was ok, probably due to the rocker shape. Perfectly fine turning on the board. Also I've ridden the Rome Shank which is a bit wider than normal, same thing.
I want to get on a really narrow board to see if there is a difference. Two would be the TRS Slim or the Yes TDF.