The EPIC Storm is a pure combo deck racing to resolve Tendrils of Agony or an Echo of Eons loop, with almost no interaction with your board, so this matchup comes down to who combos off first. The extra Unmask lets you proactively strip a key piece like Lion's Eye Diamond, Burning Wish, or a Dark Ritual before they go off, which matters since you otherwise can't interact with their combo. Silent Gravestone comes in as a cheap, low-impact artifact, while Poxwalkers and extra looting are cut in favor of faster, more proactive disruption since this is a pure race.