It was flown by an employee of the manufacturer. My bad for assuming they would know what they're doing. Clearly, they wrote the AFM. In this specific case he got a number wrong from what he was actually told, and was leaning (300hp) engine to as low as 76lph at full power.
No damage done, but looking at the EDM data you can see the confusion as mixture goes down, then back up as CHTs shoot through 400°F, then down again... and so on.
Yes, the online APS course is magical.