On any given creature the combo isn’t particularly effective, but when paired with creatures that can tap for mana, you have something special, letting you create an arbitrarily large creature by activating the Umbral Mantle for the steeply discounted price of one mana. Rather than use things like Noble Hierarch or Birds of Paradise, this deck opts for creature-lands as the last part of the combo, a wise decision for two reasons.
First, creature-lands take up land spots in your list, letting you load up heavily on them (this list has eleven) while still having space for plenty of cantrips and disruption/protection spells that combo decks rely on. The high count of creature-lands lets you mitigate the downside of being ostensibly a three-card combo deck and choose when you want to expose the most vulnerable part of your combo.
Second, those creature-lands open up a second angle of attack for the deck: Polymorph. The only creature card in the deck is Its Noodliness: Emrakul, the Aeons Torn. Once you get to five or six mana, you can animate a land and turn it into the game-ending monster for a more streamlined kill.
Because both combos require a significant mana investment on your own main phase, I like the decision to include the cheapest possible disruption: Spell Pierce, Dispel, and Disrupting Shoal so you can effectively fight over your combo the first time. Reloading by finding another creature is easy enough, but to finish the game, you need it to attack, so it can be slow. Reloading by finding any other piece is a lot more difficult, so winning on the first attempt is more important here than in other combo decks.
I didn’t even know this combo existed before seeing this deck. But Modern constantly surprises me and I’m a sucker for a nice two-card combo, even if it takes a little effort.