If you read the entire article from Stephen Phinney he points out clearly that when subjects are switched to a VLCD(Ketogenic Diet) during a short 1-2 week study they do have a loss of energy and Vo2Max volume during the first week or two. Stephen also points out that Keto adaptation takes as long as 4-6 weeks to take place and these studies did not last that long. In the studies that lasted longer the test subjects were back at their beginning energy level and Vo2 volume while being at a lower body fat%. The subjects that remained on the VLCD for an extended period of 6 weeks or longer became Keto Adaptive and had no loss of performance output.These subjects also had the weight that was previously lost added to them in a backpack during their final test.
This was also backed by Lt. Frederick Schwatka from his journal and documentation. Schwatka lived with the Inuit Indians and adopted their eating habits of a high fat diet for the entire time his expedition lasted while traveling over 3000 miles on foot. Frederick Schwatka stated "When first thrown wholly upon a diet of reindeer meat, it seems inadequate to properly nourish the system, and there is an apparent weakness and inability to perform severe exertive fatiguing journeys. But this soon passes away in the course of two or three weeks."