This is a misunderstanding of Carnot cycle efficiency. The thing that makes the Carnot cycle special is that it is reversible. So in principle, you can get 100% of the mechanical energy that you put in back out again, by running the process in reverse.

The usual efficiency calculation for a Carnot cycle tells you what fraction of heat energy that's removed from the hot reservoir can be converted to mechanical work. This is always less than 100%.

However, when running the cycle in reverse as a heat pump, it is possible to transfer more than one Joule of energy from the cold reservoir to the hot reservoir per Joule of mechanical energy used. That's why the round trip efficiency from mechanical energy back to mechanical energy can be 100%.

I have no idea what the practical efficiency of this system can be. But it's wrong to immediately reject it on theoretical grounds.