It seems almost too good to be true.
You have hit the nail on the head.
The most obvious problem that is not even signalled in the video is - where does the energy to run radio frequency generator comes from? They don't BURN salt water, they use energy to decompose it, so that produts of decomposition ca burn. And how much energy is consumed in the process, before heat evolving from the burning hydrogen/oxygen mixture can be used for running the Sterling engine? And finally: won't it be more efficient to use this energy DIRECTLY in electric motor?
That's assuming that it is not a hoax. Water decomposition using radio frequencies doesn't sound possible, taking bond eneriges involved into account. Salt water decmposes, but if you put your hand into the beam nothing happens? Well, that's strange, as our tissues are basically mixture of proteins/fats in salt water... And so on.