DoD believes there is no chance (0%) China would agree to any nuclear treaties until they've hit parity with US/Russia in raw warhead count. And btw, DoD's unclassified assertions about their warhead count is based on old assumptions of their fissile material production and plutonium/tritium availability. In terms of ICBMs, they have production facilities that can make about 90 Peacekeeper-class missiles a year.
In other words, this is less about what China wants and more about where this puts the future nuclear posture of the US.
The Danes would have allowed all those things without the annexation, think new sub pens for our Virginias, Space Force base expansions and so on. There is simply no need to piss off an ally with this nonsense.
Anthropic probably doesn't have the independent capabilities to perform a full definitive attribution of sophisticated cyberattacks. They likely detected misuse of their tools and then worked with/provided information to the intelligence community (who are familiar with the modus operandi of Chinese APTs) who then did the attribution.
It doesn't really matter how advanced your supercomputing infrastructure is .... the simulation is as good as the input and data from actual tests.
The big question is whether China is confident enough with the data they have from 47 tests.
Any non-subcritical testing is a gift to China as they are severely lagging behind US on number of tests conducted and therefore amount of data collected about warhead design.
Resumption of tests would add fresh data to verify new warhead designs over the decades since the last test. US would have a lesser need for new data given the amount of testing done during the cold war.
If US does conduct a nuclear test I bet a whole slew of test from China would come very shortly after that. Work has already been noticed in recent years in the test tunnel.