I can’t access the previous paper by the authors that contains details about the diets.

That being said, people don’t eat macronutrients, they eat foods. With equal caloric intake, a vegan diet is going to be higher in fiber and lower in fat and protein than a diet that includes meat. That’s probably part of why it’s healthy, but the reason for doing this study is to understand the mechanisms in more detail, rather than just claiming that one is better than the other.

When you say they should have supplemented fiber, you’re also implying they should have had less meat. According to the linked paper from this year, the meat-rich diet included at least 150g of meat per day, which is just a bit more than a chicken breast or a single burger patty. Probably in line with what many expect from a balanced meal.