Per the article the prototype was flow at around 20,000m and they’re specifically going up there for the efficiency reason. The design also has a “high lift-to-drag ratio” that is apparently helping out.

This one has some more detail, no way to confirm its veracity however. https://theasialive.com/chinas-yunxing-jet-completes-test-flight-eyes-supersonic-commercial-travel-by-2027/2024/10/29

It could all be marketing hype, but there are a couple other companies in US and UK working on similar projects.