It is worth noting how thin the evidence is for these claims. Here's the key sentence:
"The concerns stem from the high tempo of activity at China’s Lop Nur test site, extensive excavations at the site, and Beijing’s purported use of special chambers to contain explosions." https://twitter.com/WSJ/status/1250472018530664448

US, Russia and China all conduct subcritical tests. Subcritical tests are not prohibited the CTBT, which is not in force and which neither the US nor China has ratified. From satellites and seismic stations, subcritical tests are indistinguishable from low yield nuclear tests.

This is really a matter of belief. If you think the Russians and Chinese are up to no good, well, I can't prove they aren't. But it is also a non-falsifiable belief and the kind of thing that has led us to mistake an earthquake for a Russian nuclear test.
https://armscontrol.org/act/1997-10/press-releases/cia-says-seismic-event-near-russian-test-site-not-nuclear-explosion

You can look at the test site yourself. It is located at: 41.699° N 88.367° E

I don't know about a "high tempo" of activity, but images from @planetlabs do show China excavating one of the tunnels over the past few months. You can see a "spoil pile" grow over time, which is the earth excavated to make the tunnel dumped outside. https://www.planet.com/stories/china-excavation-at-lop-nor-4bhuLmqWR

This could be for any number of purposes: subcritical experiments, secret low-yield nuclear tests, or resuming full yield nuclear tests. The Trump team has talked about resuming US nuclear tests. If that happened, I don't imagine Xi would be far behind. https://www.nytimes.com/2016/12/27/us/politics/trump-rick-perry-nuclear-weapons.html

We had this same conversation in May 2019 when the US leveled the same accusation against Russia. I wrote a whole thread about it at the time. We could negotiate test site transparency measures with Russia and China if we wanted to solve the problem. https://twitter.com/ArmsControlWonk/status/1133814708673208320?s=20

But we won't, because the allegations suit certain people better than doing something about them. Because doing something about them would be arms control and, last I checked, this Administration hates that.