I think it's careful to look at what the study's actually said in articles such as this.
"In summary, later bedtime on weekends was associated with shorter sleep duration and poorer glycemic control in patients with type 2 diabetes. It is likely that patients with later weekend bedtimes curtail their sleep by waking up earlier."
It doesn't seem unreasonable to posit that the people going to sleep later on Weekends, were also drinking more alcohol, for instance.