That's just the same issue we've talked about a million times in this thread. The study uses eleven different controls ("graduates’ occupation, economic sector, hours worked, employment status (having multiple jobs as opposed to one full-time job), months unemployed since graduation, grade point average, undergraduate major, kind of institution attended, age, geographical region, and marital status"). Mostly, it's just demonstrating that the things it controls for are why there's a gender pay gap -- and most of those differences are still a result of sexist culture. Several of those controls also mean they're not looking various forms of discrimination: in hiring, in promotion, in those driven away by a hostile work atmosphere etc.
It's also notable that they still found a 6.6% gap overall despite using eleven(!) controls.