Romans never adopted the stirrup
...and even if they had, why are stirrups on a horse pulling a chariot?
shareI think it's called an 'intentional goof' in that it was an anachronism but necessary. While Russell Crowe is an accomplished rider the scene called for him to brandish and 'use' a sword in a rather lively fashion. The stirrup became popular in the middle ages to allow heavily armoured knights better balance in combat, lightly armoured Roman cavalry would have had years of training without them. In other words maybe the extraordinary activities Crowe would be performing in the scene called for a balance aid. That's just conjecture on my part but I can't think of anything else.
Hey! You're not old enough to drink! Now go and die for your country!!!