I think you've identified the reason: the US war planners see advantage to some "limited" nuclear exchange and are already flirting with the idea of it (not sure what gives them the confidence that it will stay within their limits if they escalate to nukes). The temptation is going to be irresistible if things start looking bad for them (and I think in a more conventional war China enjoys an advantage here purely from geography).

Even from the Chinese perspective, once they're in an active war that threatens their basic territorial integrity, who knows if they can really stick to NFU.