I’m not an expert on Blender too, but from what see I assume that they do it the same way as in the soft body sample provided here: they use Bullet SoftBodyHelper to DERIVE a tetra mesh from a convex hull at RUNTIME.
In Blender this happens behind the scenes. Blender users never see the tetra mesh, and as far as I know there is no way of exporting it. And I don’t know any file formats for storing a tetra mesh (besides “proprietary” or “custom” stuff like e.g. PhysX uses).
Sidenote: PhysX has a small application which is also deriving a tetra mesh from an arbitrary triangle mesh, and then saves the tetra mesh to a file (binary or ascii). Then a PhysX game just LOADS the saved file at runtime, which is obviously much faster, and allows better optimization of the tetra mesh.