In your climbing days what did you carry to eat? Have your opinions on this changed? What about other endurance types you have in your gym how do they get by?
The writer's premise was that gels make people sick, but the tea and biscuits (I surmise he is English) he used during a military work-up were tolerated just fine. He cited something I wrote on the Church of the Big Ring site regarding stomach issues related to gel as well as anecdotes from other friends who had similar experiences.
Eat for the intensity of the output: at lower intensity/ HR you can tolerate more complex foods, at higher intensity/ HR only less complex sources will empty from the stomach and be assimilated without compromising circulation, muscle function, thermo-regulation, etc (by robbing blood from circulation to facilitate digestion). So in the case of hard effort only liquid and gels are recommended. When you are going easy, at an "ultra" pace eat what you want and have proven you can tolerate.
This has been studied exhaustively and the best rate of intake and composition is more or less known. However, there is some individual variation to digestion and further detours for personal taste/ preference, the motion of the sport, i.e. less sloshing on a bike, more sloshing on foot, etc. The final point really is practice, practice, practice. Rehearse in training how and what you will eat on the day so there are no surprises.