I'm not sure why they were surprised by this. NASA has recorded time manipulation before in regards to objects with mass.
The more dense an object is, the more effect it has on time itself. Time dilation is constantly seen between Earth and the ISS. The easiest way to understand it is to hold a blanket tight from all four corners. Then take balls of varying mass and roll them onto the blanket.(baseball vs bowling ball) The way the blanket bends around the objects is exactly how time bends around objects with mass. The more mass the object has, the more it affects space time or in our case, how a bowling ball affects the blanket.