I'm sure there's significance to the horses walking in front of the procession too- they admitted there was no precedent for it, but the royals wouldn't change it- even when warned of the potential optics (droppings). It seemed like a deliberate decision and from the photos the car was surrounded by it. One last half-thought. Only one former act that I'm aware of involves being 'drawn' through the city behind a horse and it hasn't been done in a long time (it still won't have been, having been phased out, but I did wonder if it was a subtle nod)