Such a phase change, (liquid to gas), can carry large amounts of heat away from hot components, but suffer from at least two drawbacks.
1. Once the coolant is in its gaseous phase it becomes a rather efficient insulator, preventing liquid coolant from having maximally efficient contact to the hot component.
2. The collapse of the bubbles of gaseous coolant can cause the erosion of the surfaces adjacent.
A viscous coolant which phase changes to a more fluid liquid solves both of those problems.
I don't know if that's what they're doing but it's a possibility.