What I'm guessing happened was: he got briefing on how badly Sentinel is doing and how they can't afford it, he also got a briefing or already know how their 1970s Minutemen missiles are mostly defunct, i.e. America is already mostly de-nuclearized, so he figured instead of going broke trying to replace Minutemen, why not negotiate disarment where we get rid of our non-working nukes in exchange for China and Russia getting rid of their working ones.
It's always been a theory that US completely neglected nuclear force maintenance after convincing themselves that history ended in 91, then you got US Strategic Command head saying America is a "sinking ship", how everyone who worked on Minutemen are dead or retired, then Biden restarting B61-13 gravity bomb development and Congress funding Sentinel at insane priority levels, in a way the fact that Trump even floated the idea of denuclearization is almost a final admission that US nuclear stockpile is mostly for show at this point.
Of course Russia and China aren't stupid and will play accordingly with this information.