Mono Green Landfall floods the board with creatures like Mossborn Hydra and Mightform Harmonizer, so adding a third Deadly Cover-Up (alongside the two mainboard copies) gives us maximum sweeper density to reset the board. Disdainful Stroke counters their bigger payoffs with mana value 4 or greater, such as Mightform Harmonizer, Icetill Explorer, and Sapling Nursery. It can't hit cheaper threats like Mossborn Hydra or Earthbender Ascension, which are mana value 3. Duress is weak here because most of their threats are creatures, which Duress can't take, and the noncreature spells it can hit are mostly ramp and value pieces rather than must-answer cards.