The key word there is marketed. Most all of that is marketing hype for the century crowd. You can make a full race geometry frame just as compliant as a "endurance" frame with the proper use of frame materials. This is especially true since the advent of carbon as a frame material. Just because a bike has more relaxed steering geometry or longer seat stays does not necessarily mean the bike is going to have a softer ride(look at a touring frame). Usually longer seat stays are to allow the installation of fenders and more relaxed steering provide a little more stability, neither of which is a requirement for endurance riding. The stability is the reason it is popular with many racers who race in events like Pari-Roubaix and are racing on cobblestones where it's useful. Notice that they go back to their "regular" race bike for all other events. The relaxed geometry is also useful if you are carrying a load on the handlebars. The taller head tube wouldn't be needed if every bike shop in the country didn't lop off the steerer as soon as a new bike come in the door.
Sorry but I really think those Zertz inserts, crooks in the seat tubes/forks, b-stays etc are just gimmicks.
Have you ever been to a real endurance event? Like the Gold Rush, Furnace Creek 508, Race Across Oregon, Race Across America etc. See how many "endurance" bikes there are there. Not many.
Now, the Volagi interests me. I do like some of the stuff they've done with that bike. I haven't had a chance to ride one yet but, I am looking forward to it.
I apologize for baiting you a bit here. I've been doing "endurance" cycling events for more than 20years and I've seen fads come and go. I see the "endurance" bike as just another fad. Most people (with the exception of the Volagi) those bikes are marketed for are people who think a century ride is a life time event.