China claims new airship can detect stealth US F-35 1,240 miles away

Infrared is a line of sight kind of thing and 20km altitude ain't mathing with 2000km detection distance. At 20km the horizon is only like 500km. So either these things are way higher, like above the Kármán line high, or they mean they can spot the jets anywhere they can physically get line of sight on them and the detection technology isn't the limiting factor.