Fast travel is essential, I guess what they meant is this: you are on the horse, an enemy appears, you have to get down from the horse to fight, then get back on, then you spot some alchemy ingredients you want to collect, then get off the horse, collect them, then get back on, then you walk some more and see an NPC, then get off the horse, talk, get back on...
Since you can only basically move with the horse, the constant getting off-on really interrupts the flow of wandering the wilderness, and the resources needed to actually flesh out horseback mechanics (talking, fighting, interacting) where not worth the investment, so they left horses in their most basic (and honestly silly) iteration.