They’re equipped with a giant mace designed to smash shields and send kinetic energy straight through heavy armor. The game acknowledges the strangeness of this ordeal even in the description of the unit:
While the mace doesn’t at first appear to be a weapon suited to fighting cavalry, used with precision, the powerful strikes can smash through armor to crush the legs of horses and knees or riders. Only the heaviest armor could turn aside the attacks of the Katapeltes - the shield smasher.
Could they not be sent to the anti-infantry role… or given some weapon that reflects their role better instead?