Izzet Phoenix wants to chain cheap spells to enable Arclight Phoenix recursion, so tempo-positive counters that stop their spells on curve are far better than top-end cards like Quantum Riddler. Spell Pierce is excellent against their many one-mana cantrips (Consider, Opt, Sleight of Hand) and Change the Equation counters most of their red instants/sorceries outright since they're mana value 6 or less. Boomerang Basics is too slow against a deck trying to combo off quickly, so it's trimmed for proactive counterspells that disrupt their engine before Arclight Phoenix starts coming back. Race carefully and hold up interaction whenever possible since they can rebuild fast.