Not entirely true. The hybrid and diesel systems are answers to two different questions: Diesel is very efficient at a constant speed. Hybrid doesn't offer much there. A Prius on a long trip will use about the same amount of fuel as a comparable car with an efficient regular engine (because that is what it uses). A diesel engine will typically use quite a bit less because it's a lot more efficient. From personal experience modern diesel engines also seem a bit more forgiving about higher speeds. You can still get decent fuel efficiency at Autobahn speeds.
Hybrid allows you to save fuel because it can recoup energy during braking. A regular car will convert all kinetic energy into heat through the brakes when slowing down. A hybrid car will put part of that kinetic energy back into the batteries. So a hybrid is good in stop and go traffic. A diesel doesn't help much there as it can't recoup the energy.
So a hybrid diesel seems to offer the best of both worlds: efficient on short and long trips. PSA (Peugeot and Citroen) sell a diesel hybrid: 3,8/100 km, 147 kW (200hp). The PSA Hybrid4 models definitely look good on paper, but I have no real world experience with them.