[jniedrauer]

That is the normal state of ultra endurance athletes during long runs[0]. A lot of training stimulus is fat adaptation. Coaches will tell you that you begin to exhaust your glycogen stores beyond the 90 minute mark, and I can tell you subjectively that the body switches over around then. I have done fasted runs up to 3 hours before. Runs of that duration are physically impossible without burning fat as fuel.

[0]: https://www.tandfonline.com/doi/abs/10.1080/02640414.2019.17...

[phonypc]

The full text of your link shows the athletes didn't have elevated ketones until the second day of the race. After 80km, or about 12 hours of running, on an energy deficit.