Note that for a year the subjects were on a "maintenance diet", which most probably was intended to limit their calorie intake to match their daily energy expenditure, and still gained some of the weight back.
I believe a more exact wording would be: "the subjects were asked to keep a maintenance diet, and they reported doing so". Without reading the article (which is not freely available), it's hard to see how they verified the subjects actually kept the diet for 1 year.