With all these big, fat butts, Lightning Bolt isn't going to save you.
You might think that a Mono-Green Aggro deck simply can't survive in Modern due to all the combo decks, but this deck has a surprising amount of interaction for a mono-green deck. If they're trying to put Splinter Twin on their Deceiver Exarch, you have Beast Within, Dismember, and even Vines of Vastwood to stop it. Overall, that's pretty impressive.
Against anyone else, you get to pressure them with early, efficient creatures, some of which are very difficult to kill. Cards like Scavenging Ooze, Thrun, and Garruk allow you to have some presence in the late game. Rancor provides a constant way to push through damage. I could see having more Treetop Villages to do the same.
It might look like a pile of random cards, but 4-0ing a Modern Daily Event is pretty impressive for a deck that looks so weak on paper. In reality, there's a lot of play to this deck, and it is very well-constructed. I doubt that Mono-Green Aggro is the next big thing, but it seems like it's about as good as something like G/W Hatebears. Those types of decks certainly have their bad matchups, but can thrive in certain metagames.