Nvidia doesn't licence its designs to other companies for incorporation into their own chip designs, ARM does. That is the main distinction I think, and why people want it to be a neutral third party.
IIUC, part of the deal was that NVIDIA would license its designs via ARM.
They say that now... It could even very well be the case. However, there are plenty of ways to manipulate those licensing agreements to work more in Nvidia's favor than they do today.
They've been saying that from the beginning. There are billions of ARM devices, the ability to sell a GPU to each is worth money.
Ability to forcibly sell a gpu to each is worth even more money (ie. see lawsuit on how Quallcomm bundle their modem on every SoC for a premium)