Yea, Microsoft is correct. You have a gas inside, or a drop of liquid. It is the same technology used in laptop heatsink cooling, and most aftermarket heatsink solution for desktop CPUs. If you cut the pipes it will be empty as well. Different manufacture cools the technology by different marketing names. A popular one is; "Vapor Chamber".
Regardless of the name, it all works the same. At the end of the day, it still a brilliant idea of having a heatsink on a hot mobile chip (and do it well). The issue with this technique, if you want to be technical, is that you have no fans or vents or something to cool the heatsink, so only delay the inevitable of having a hetasink match the CPU temp, and then it throttles. But regardless, it a normal day to day usage, it avoid experiencing throttling unlike other phones on the market with the same chip.
I am wondering if Microsoft used this to test the system in a real world situation, for future phones. Maybe Microsoft is considering an Intel Atom or Core M chip in the Surface Phone, to allow Win32 programs to run under Continuum, and really have Continuum have a nice performance increase as well.