But have you looked at that site? They're not talking about the 1-2 hour trail rides. These are riding holidays, most of which are 5-7 days long (the minimum seems to be 3 days). That could be why they are a bit more, errrr, emphatic about rider fitness.
Exactly that^. This company organizes SERIOUS riding tours, where you are riding 6-7 hours a day for sometimes two weeks at a time. Many of their trips cross some strenuous terrain and involve quite a bit of fast paced work. A rider needs to be quite fit to keep up with the group over that period of time.