None of the Mount Equipments work on lvl 20 characters despite them stating they are for lvl 20+.
I have noticed this also (and I have been able to apply the waterwalking equipment to multiple toons, so clearly I have it unlocked on my PTR account).
Also, what doesn’t make much sense is that whatever Mount Equipment you choose to place in the Mount Equipment slot applies to ALL of your mounts. Furthermore if you replace the Anglers’ Water Strider with say, the Comfortable Rider’s Barding, it destroys the Anglers’ Water Strider and now you have the Comfortable Rider’s Barding on ALL of you mounts. Why can’t these Mount Equipment slots be independent?