It is certainly possible to do 3D graphics in Flash. You even get hardware acceleration if you use the no longer new Stage3D subsystem. They can just export the models as models and let your expensive graphics card do the rendering. They could possibly even use a cross compiler to use the actual game engine in Flash.

But they know that they could just make a plain old downloadable PE file with the game demo. I am confident that they have an existing pc port of the engine. It is a pain to do embedded development without a pc port.