1) How do you know biology hasn't changed? The human gut microbiome has changed wildly over the past 50 to 100 years due to changes in modern diet.

2) Another point, the article is talking about the biology of people who have already been obese and then lost the weight. Given that people have more disposable income and obesity trends have risen, the inability to lose weight only after you first become obese would help explain why things like childhood obesity have negative impacts for the rest of a person's life.