The source article says they would be used to detect Chinese and Russian missiles. But that doesn’t mean the US had any plans to just float them over China or Russia.
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 them? 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.
The context is that US is developing high altitude balloons for detecting hyper-sonic weapons. Developing a technology and actively using in another country's airspace are quite different things, so it's not really that surprising that articles don't find it important to include.
I present to you Tu-128, the USSR's dedicated NATO balloon buster [0]. US found it quite funny to send in hundreds of balloons to reconnoitre and send a message[1][2].
US Military to Use High-Altitude Balloons Against China, Russia: Report
https://www.thedefensepost.com/2022/07/06/us-military-balloo...