When making potentially very high stakes claims about violations of the global nuclear test ban treaty, one would expect a high evidentiary standard. This is not the case with the US, however, which is now accusing China of maybe violating the ban, and which is offering no evidence at all.
The whole speculation from the State Department report is based on unspecified activities at one of China’s nuclear sites, and general distrust of China. This would of course not prove that any tests took place.
The Comprehensive Test Ban Treaty (CTBT) sets a standard of zero-yield, meaning no nuclear reaction set in place by explosives. Global monitoring means any test with actual yield of any meaningful amount would be detected, making the US claims even weaker.
China is moving toward modernization of its program, which is worrying the US, but they already have a substantial nuclear arsenal, and it isn’t at all clear why the US would imagine they even need to do such testing.
Making such allegations, however, seems to be popular with the US in recent years, however. In 2019, the US similarly claimed Russia was “probably” violating the CTBT, and again offered no evidence.
As with China, Russia’s mastery of the nuclear cycle is established, and has been for decades. The US has yet to come up with a good reason why either nation would even feel the need to risk a treaty violation.