That would be true if we were talking about series hybrids.

But every hybrid you can buy these days is either parallel (most) or series/parallel (toyotas). These engines are not exclusively operating in the sweet spot. Most of the benefits you get from hybridization these days come from regen braking and being able to use a smaller engine (the electric can help out during hard accels, so you don't have to size the engine to be able to handle the max output you desire, it only has to handle the max steady state output).

Also you forgot to mention that diesels have a higher temperature combustion chamber. That gets you a lot more juice from basic thermodynamics...