How is it water at that temp to begin with? That is a damn hot space to be in. Plate tectonics anyone?
From the article: “The reservoir, which is said to be three times the volume of the oceans on the surface, is contained within highly-pressurized rock known as ringwoodite.” So it’s not “an ocean” but contained within rock. Ironically enough, volcanic magma depends on water for its molten state. That’s why so many volcanos are found adjacent to plate subduction zones. I agree it seems counterintuitive but matter and materials behave differently under great heat and pressure than they do on the surface. The granite you may have in your kitchen sink requires water to form and the speed of cooling determines the size of the included crystals. “Water, water everywhere and not a drop to spare.”
It’s chemically-bound water in solid form in a substance called Ringwoodite. It doesn’t pour or flow like water.