There were 2 of these 10Kw units required to cruise at 75Mph, so the efficiency, assuming 35% thermal efficiency, is 22mpg.
Given that they're small and easy to maintain, perhaps that doesn't matter if they're only backups, or if this is just a first-iteration technology that may get substantially better.
The big concern imho, is vibration. Unlike a crankshaft-based engine/motor, there is no physical coupling of the pistons if you deploy two of these in a horizontal configuration (as TFA suggests would counter vibration).
The lack of coupling means that the pistons are not mechanically synchronized, which means they don't create forces which act against each other.
I'd have to imagine that one could approximate the physical coupling by varying the timing and mixture, but.. I have no idea how actually effective that'd be.
Anyway.. vibration. Big deal.