Are you sure it's supposed to work? Only played for a while Yesterday, but map after map horse archers were topping the scoreboard and getting valour for their efforts. They had no problems shooting at me, running in 68 body armor - do I need more?
...every arrow where hits an enemy player & or his/her horse counts the score. so ..no wonder with an amount of 30+ arrows , an good survivability with skills (to dodge ex.) & a horse.
you can test this with an stf ... take a horse , 5-6 pt , rest riding & wpf , 4 stacks throwing stones, snowflakes or daggers , aim & just spam. (s)kills & damage doesn't really matter for scorewhores.
(same goes for siege btw ..just spam out the hell in melee, doesn't matter if you're surrounded with helping teammates. some dudes got most of the time incredible scores with that "tactic".)
note: ...nothing against ha's & their skills (more or less) in general & personal , i know there are very good & gentle ha's out there since a long long time.