[ThePhysicist]

Yes, seems like a very misleading or even outright wrong comparison to me as well. Combustion engines are not much heavier than electric engines if you only count the motor block. And if you look at motorbikes (which have probably the most compact engines) their power/weight ratio should be even higher than that of an electric engine.

That said, the limiting factors in terms of power per unit of weight are probably the conductivity of the wires (as the generated heat must be dealt with) and the magnetic force that acts on the motor during its operation (actio=reaction). In a combustion engine it's probably the ability of the cylinder walls to withstand the explosion pressure.

Concerning high power densities, I'd look at electricity generator turbines as well, as they are basically reverse motors and should have a very efficient power to weight ratio (e.g. several MW for a few tons).

Would be great to see a detailed comparison of these factors, which the article does not provide unfortunately.

And, like other people have already pointed out, what really matters in most circumstances is the combined weight of the motor and the energy source. Here, gasoline unfortunately still beats battery-based energy storage by a large factor.