With a light lance you can outreach everything except a pike. It all depends on three things: When you released your stab, when he released his(or couch in this case), and footwork.
If you want to give yourself more wiggle room, move backwards as you stab. Aim for the horse's head too, not the feet. Feet do more damage, chest hits the player, and head will rear the horse fastest. Dying is bad, so in the case they have such a long weapon just get the horse to stop.