Manufacturers can call their IC pins whatever they like.
NC stands for "No connection", as the datasheet even says. There is nothing wrong with that.
The only questionable part is "Must be connected to ground" for a pin that is not connected to anything. Without further explanation from the manufacturer (did you read the whole datasheet carefully?), we don't know why they say these pins must be connected to ground. It doesn't matter though. They've clearly said what you need to do to stay within spec. They don't owe you a "why" for something as unambiguous as "Must be connected ground".
Perhaps they just don't want the pins to pick up noise, then bring that noise nearer the die. If they explained that, then they'd get the inevitable questions about whether it's really necessary, when you can get away with it or not, etc. Simply stating that they must be connected to ground gets around all that time-wasting.
Perhaps these are actually pins used during manufacturing test. In that case, they are merely presenting them as "no connection", meaning "not something you should ever connect to". As before, there is no upside in providing more detail, so they just say these pins must be connected to ground.
In either case, they have clearly told you what you need to do, and covered their butt so that if you don't follow the requirement, it's on you if the chip gets damaged or doesn't work. Trying to split hairs about what they maybe should have called these pins is pointless.