I don’t see much of a difference between this and current technology. NASA has been using nuclear reactors in probes and rovers for quite some time. Presently deep space probes use nuclear reactors to generate electricity. Propulsion is produced by using electricity to accelerate ions. The ions come from gas stored in cold liquid form.
I don’t see any breakthrough technology here. From what I can tell reading the article, it’s just a lightly different way of creating propulsion, pressurizing the gas with heat instead of accelerating it with an electromagnetic field. Seems like a step back actually.