A lot of people with sedentary lifestyles will gain weight on a 2000kcal diet.
And the heavier you are, the easier it is to lose weight because it takes energy to just maintain all that extra body weight.
Point being, there are certainly some people would not lose weight on 2000 calories/day, but I would guess that isn't true for most of the people in this study.
Well, the higher a calorie intake you are likely to be able to maintain and still lose weight, which is very much not the same as “the easier it is to lose weight.”