First of all, START isn't even about non-proliferation. Has nothing to do with it. It's a series of agreements between two superpowers that served mostly as a diplomatic tool to de-escalate Cold War tensions and provide political cover for military budget reductions. It's been wildly successful in that regard.

The actual Nuclear Non-Proliferation Treaty is entirely about non-proliferation, a pre-emptive attempt to keep the capability of nuclear warfare out of the hands of most nations. It has absolutely nothing to do with preventing nuclear warfare between those nations already possessing them.

We haven't had nuclear war because so far, nobody who either actually wants to end the world, or is stupid enough to think he can "win" a nuclear war, has been in control of nuclear weapons. Not because of some piece of paper saying "I promise I won't kill you". Hilter had one of those with Stalin, how'd that work out?

"Cyber warfare" doesn't have the problems of nuclear warfare. It's unlikely to end the world, it's relatively cheap, the tools are widely available, and unlike physical warfare which destroys economies, it's actually about the economy. It's about getting a leg up on the competition. It's about money, period. The world some non-technical people live in where "cyber warfare" is all about winning actual wars is a world of fantasy.

In human history, only one thing has reliably stopped wars, and that is the tying together of nations' economies so closely that war between them becomes economically impossible. This just doesn't work in "cyber warfare".