The NSG debate is a good primer of the manner in which world politics functions. The NSG itself is not a body based in international law, but a cartel of the powerful — in this case, countries with the capacity to conduct nuclear trade. The only language in which it communicates is power; and the only method of negotiation is give and take.
There are other similar bodies, beginning with the G7/G8 — now somewhat chastened — but which once acted as arbiter of the international economic system. So there is the Missile Technology Control Regime (MTCR), a club of countries which have the know-how of making missiles, space systems or their components.
The Australia Group is a cartel of countries making chemicals and the precursors of chemical weapons — and then there’s the Wassenaar Group of countries with advanced conventional weapons technologies.
As part of the India-US nuclear agreement of 2008, the US promised India ease of entry into all these groups. This was said to be huge for India, as the only country that could achieve this goal was the US, the sole global superpower.
Being cartels and not international agreements, these regimes are not always universal. China, the major missile and arms exporting power, is not a member of the MTCR or the Wassenaar, though it claims to harmonise its rules with both of them.
Given this perspective, China’s formal position raising the issue of the Nuclear Non Proliferation Treaty was a red-herring.