By the same token, I’d ask you to find the science that says any kind of wrap we can put on a horse’s leg (one that allows the fetlock to flex at all) provides meaningful support. IMO, the forces exerted on the suspensory apparatus as well as the flexor tendons swamp the effects of any kind of compression you can create on the leg.
Otherwise, OP, I think polos are used primarily to draw the eye to the lower part of the leg and make horses look like they are more extravagant movers.