Amulet Titan wants to chain bounce-lands and Amulet of Vigor into a fast Primeval Titan or Scapeshift kill, so Disruptor Flute naming Primeval Titan or Scapeshift makes those key payoffs cost {3} more to cast. Flute only taxes casting, so it does not stop Green Sun's Zenith from putting a Titan straight onto the battlefield. Grafdigger's Cage is a cheap noncreature artifact that stops creature cards from entering the battlefield from libraries and graveyards, which shuts off Green Sun's Zenith fetching Primeval Titan or another green creature. It does not stop Summoner's Pact, which puts the creature into their hand, and it does not stop Primeval Titan putting lands onto the battlefield, so it is a narrow, low-risk piece of interaction. We are both big-mana decks racing to our respective payoffs, so we trim our slowest, most replaceable cards (Nulldrifter, a Devourer of Destiny, a Sowing Mycospawn) for cheap proactive disruption rather than diluting our own speed.