Selesnya Landfall wants to go wide and pump via Earthbender Ascension and Mightform Harmonizer, making Split Up a strong sweeper against their board of mostly untapped blockers or attackers depending on the turn. Twinmaw Stormbrood is the easiest cut: its 5/4 flying body costs six mana, which is too expensive for a plan built on a low curve, and we'd rather spend the slot on a sweeper than a top-end finisher. We keep our cheap burn and low curve intact since beating them down before Ba Sing Se and Earthbender Ascension take over is the correct plan.