If someone is transitioning from a high carb diet, it's pretty normal to see bigger drops the first few weeks/month, due to a mixture of fat loss and water loss. I was already a pretty lean weight when I transitioned to low carb, and lost around 5lbs the first few days, and that was just reducing carb intake to around 100g net carbs a day, so not even really low carb.

For someone who's heavier, a 10lb or more drop in the first few weeks seems pretty typical. Fung is cashing in on that by claiming his plan will get those results, but really any low carb diet that's greatly reducing someone's carbs, along with the calorie restricting that's also taking place with the dietary changes, will most likely produce the same results.