Fair enough. Wikipedia provides no citations for that number, and I have had zero luck finding actual published studies, which is actually a bit weird.
However, conventional diesel cars are nowhere near twice as efficient as petrol cars. I do not understand why that changes with the addition of a hybrid drive system -- the hybrid drive system should improve the efficiency of a petrol car comparatively more. E.g. look at spritmonitor.de, top diesel performer (after the special models VW Lupo 3L and Audi A2 3L which do not have equivalent petrol cars) is the Citroen C1: 4.2 l/100km on diesel, 5.2 l/100km on petrol. Less than 25% improvement, and half of that is just because diesel is heavier.
Anyway, I will be convinced when someone builds a small diesel range extender. I won't be holding my breath.