What I understood from the article: it's not the engine that rotates.

Instead, the fuel mixture ignites in one spot, and the explosion spreads at Mach 5 around the engine but once it reaches the end of the combustible fuel, more is injected behind it so the explosion can just keep traveling around in circles.

Judging by the article they managed to get that working, and also came up with a better way to take videos of this happening (high speed camera, tracer chemicals in the hydrogen, better signal strength).