BRG goes wide with cheap bodies like Gene Pollinator, Greedy Freebooter, and Rottenmouth Viper fodder, so Split Up's sweep of all tapped or all untapped creatures punishes their go-wide attacks or blocks. Seam Rip exiles early value creatures such as Callous Inspector or Umbral Collar Zealot before they generate sacrifice synergy. Smaug the Magnificent is cut because its attack damage only scales with the Treasures you have built up over several turns, which makes it slower than the cheap interaction coming in against their early sacrifice engines, while Erode trims since Split Up covers board states more efficiently against their wide strategy. The full land count is preserved to keep curving into Sarkhan and Hellkites on schedule.