It does however suggest a possible explanation that the Chinese are floating them over the US to detect tests of US hypersonics. It does seem like a good reason for it since just about everything else can be seen by spy satellites much more easily.

You raise a good point, and are right that the linked article doesn't explicitly say this. And searching a bit, I didn't find any other clear claims about how these would be used. But is there actually some other way that would make sense to deploy it? At 80,000 feet or so, I think it would only work as a sensor platform if it was close to the area you were monitoring, and being a balloon, it would be difficult to stay at an exact location just outside of a country's airspace.