Marathon runners are skinny because they spend all their free time training in running and not in bodybuilding. Plus bodybuilding does not necessarily help make them become better runners.
Bodybuilders can't run because they spend all their free time training in bodybuilding and not in running. Plus endurance running does not necessarily help them become better bodybuilders.
Alex Viada can do both because he is a professional who makes his money training in both. Plus training in both helps him sell more books, seminars, and raise his coaching fees.
Both sports take a considerable amount of time, money, and energy. Everyone has a limited amount of each. So, if you invest in one thing, then the other thing suffers.
The second issue is training. If you've ever read any of Alex Viada's stuff, he puts a lot of emphasis on recovery. The biggest mistake people make when they do a sort of hybrid training is they will take a training program designed for runners and mash it with a training program designed for bodybuliders. They will do both three weeks and then wonder why they are constantly exhausted and breaking apart.
Training for both requires a well-designed training template that incorporates enough recovery to ensure you have the energy to lift heavier weight and runner longer distances. You have to also utilize proper cycling techniques so that one cycle focuses on increased lifting and maintaining running, while the other focuses on increased running while maintaining lifting.
And finally, most people can't or don't want to eat amount of calories that kind of training requires. I don't know if you've ever tried to eat 10,000 calories a day, but it is not easy or cheap.
tldr: Marathon running doesn't eat away muscle. Not lifting, not recovering, and not eating eats away muscle.