The guy dismisses the idea of a dismount device as he seems to think many hundreds would be needed in any charge. But it only takes one horse stepping on one of them to totally disrupt a massed charge.
The holes would make them easier to roll and to break; the nobs easier to kick out under a hoof, once stepped on - like stepping on marbles for a horse.
Broken ones would be even more effective as they could cause the horse’s hoof to become cut on the sharp edges and even caught in the casing - possibly breaking the horse’s leg.
Maybe the wrapping solution you offer was why they were only used for a short period. Once used, the enemy would be wise to the ploy and find other ways to defeat such a tactic.
If the surface weren’t soft dirt, perhaps. If these were projectiles, that could be mildly disruptive, and would wind up on the ground in the middle of the charge as it continued into the rain of stuff. But rocks are free, and there aren’t enough of these to make this make sense.
Broken pottery would also work, and the Romans left a lot of that in piles wherever they lived a while. But they are known to have used javelins, arrows, even sling stones, plus their artillery (which is where the word comes from I think), which was effective against troops as well as fortifications.
The problem with the dismount idea is caltrops existed already. Way easier to make. Sharp and therefore much more effective.