From the article:

“If you were there, in this infant universe, one second would seem like one second – but from our position, more than 12 billion years into the future, that early time appears to drag.”

They are comparing to time now. If you assume a quasar expels stuff at the same rate through all time, then when you look far back in time you should see pulses coming from the distant stars at the same rate as now. Yes, that light took billions of years to get here but the pulsing rate should be the same.

They found that it isn't, it's five times slower, which implies that time then must be five times slower than now.