There are a few good posts in the discussion of that article that have some good information. I thought this explained it quite well:
The walls are lined with a wicking material that holds the liquid and carries it back to the hot part, where it boils and enters the lumen of the pipe, travels to the cold end, condenses, and repeats the cycle.
But the important thing is that, in a cold state, ALL of the liquid is held in the wick material. Any excess would be pointless. And it's held quite firmly; remember it's the capillary action that sucks the liquid the whole length of the heat pipe back to the hot end.
The methodology of their "investigation" looked to be about as haphazard as their writing, so I'm not surprised that it completely missed the mark.