This card doesn't say its effects only applies to YOUR tarot cards, just that the hand portion only applies to your hand. While players without a Moon Pendant wouldn't be able to commit Tarots from their hand (as they would have no icons while in their hand), every player would get the benefit of each other's Moon Pendants while a Tarot card is committed. In a 4 player game you could have everyone run a copy, and with all 4 out, every Tarot card would have 8 wild icons. Or, using Relic Hunter, if each player has two Moon Pendants, each Tarot card would have a whopping 16 wild icons. Which seems....pretty good.
I doubt this is intended, and you'd need multiple packs without using proxies, but if you wanted a fun cheesy multiplayer buildaround, this could be it!