They say this was detected years ago, but they chose not to even say anything about it until "a day after the treaty limiting U.S. and Russian missile and warhead deployments expired"?
I don't buy it. I do buy that China has a lot more nukes than before, there doesn't seem be to be any disagreement about that, but "revealing" years-old tests now smells more like an excuse for US build-up than anything else.
This disclosure was pretty obviously intended to put international pressure on China to conform to norms on nuclear testing, while also letting them know that the US can detect their top secret nuclear tests despite their best efforts to hide them.
I'm sure the timing of the disclosure wasn't accidental but I don't see much nefarious about it. The US doesn't disclose their knowledge of this type of thing unless there is some geopolitical leverage in doing so, since it makes it easier for other countries to calibrate US intelligence capabilities.