This is actually one of those rare cases where you could legit say "how could Biden let this happen?"
After seeing Trump 1.0 and his disregard for his fellow Americans (particularly during COVID) CMC probably figured the current level of strategic deterrence is not enough by a long shot. So they started the PLARF expansion. During Biden's term it became very clear it was happening but all of Biden's people pretended to not see it in the typical ostrich manoeuvre so they didn't have to do anything about it. Now with Trump 2.0 things are increasingly becoming harder to hide and if one day PLARF deliberately let it slip that there are 1000 warheads on station at any one time someone is going to have to explain to the American people how it happened and why no one said or did anything about it.
There's a theory that
was actually a warning to the US telling them to do something in a shitlib hanjian sort of war because it's hardly up to him to advocate for more warheads. But Biden's people either didn't understand what he was trying to hint at or knew what he was trying to say but didn't want to touch the topic with a 10ft clown pole.