The situation today is different. This engine is supposed to optimize hybrid cars (it is supposed to generate electricity to drive an electric motor) and hybrids are something new, they haven't existed for more than a century, which means that there is likely more untapped potential for optimization. This engine wouldn't be practical in a standard gasoline car and it's not designed to be used in one.
It obviously can still fail but I don't think it's possible to draw historical parallels.
Well if we're talking about that, then reciprocating engines can do pretty well (the prius engine is over 33% efficient when running in the ideal range, and it actually exists today; diesels do even better)
But maybe an electric car needs a gasoline engine as much as a tablet computer needs a keyboard, and maybe we're several years away from seeing that in the electric car case, just as the belief that a tablet didn't need a keyboard didn't become widespread until the iPad.
It obviously can still fail but I don't think it's possible to draw historical parallels.