But their missiles are not as good as the Russian / American ones and they don't have advanced submarines which are capable of launching SLBMs.
I would not be surprised if most countries that have nuclear weapons have many more nukes then they publicly admit and what they agreed to. I don't think this is something unique to china.
Countries like Israel should also be encouraged to to disclose their number of nuclear weapons, aswell as signing up for the NPT. No doubt most countries overestimate their number of nukes, partly because its become a global dick waving contest.