It is not a prohibition against riding a zamilah. Rather, it is an instruction to take precaution against falling. This is similar to the saying: "Whoever goes out for Hajj or jihad in the way of Allah سبحانه وتعالى should make a will." In earlier times, only zamilahs (simple mounts) were used, as litters and palanquins (mahamil) were a later innovation and were not known in earlier periods.