I think that the value quoted here is off by three orders of magnitude.

"We pitch 400 million cells per milliliter per degree Plato, or 40 percent of the recommended amount to pitch,” he explains. “We achieve a faster, healthier fermentation by pitching a little less yeast.”

I believe that the brewer meant to say 400,000 cells per milliliter per degree Plato. If the brewer is pitching slurry, not lab grown yeast, then it absolutely makes sense. Pitching 400,000 cells per milliliter instead of 1,000,000 cells per milliliter results in at least one additional replication event, which, in turn, results in a younger culture at the end of fermentation. The problem with repitching is that the average age of the cells in the culture increases with each new pitch. In essence, it's better to underpitch than it is to overpitch from a yeast health point of view.

With that said, let's work the math. The beer has an ABV of 7.2. Let's assume that he is achieving an AA of around 78%, which is typical with Bry 96. That value places the O.G. at 1.070, or 17 Plato.

17 * 400,000 = 6.8 million cells per milliliter

6.8 x 1000 = 6.8 billion cells per liter

If the maximum cell density for a liter is 200 billion cells, then the number of replication events necessary to reach maximum cell density is log(200/6.8 ) / log(2) = 4.9. On the other hand, pitching 1,000,000 cells per milliliter results in 17 x 1,000,000 = 17 million cells per milliliter. The number of replication events necessary to reach maximum cell density is log(200/ 17) / log(2) = 3.6. Herein, lies the beauty of exponential growth. It literally makes yeast cultures like nuclear weapons in that close is more than good enough. Other other hand, each fermentation adds up to 600,000 x 17 x 1,000 = 10.2 billion new cells per liter that would not have been created if the batch had been pitched at a rate 1,000,000 cells per degree Plato per milliliter, resulting in culture with a younger average cell age. If we serially repitch, this practice will make a huge difference in the health/vigor of the culture over time.