I'm not sure about the 50 mpg around town for the RXH - using hypermiling techniques I'd say the MB could best that.
I think the test was fair and square, and simply underlines that powerhybrdis don't get good mileage, period. They get good power at normal mileage, but that's it. Diesels on the other hand get good mileage, but are dirtier. At least RHx owners can feel good about their cars being clean, even if they do burn loads of fuel.
When I saw the headline for the article (elsewhere) my guess was indeed that the diesel MB woudl outperform the RXH in terms of mileage.