Affinity's entire manabase and most of its threats are artifacts, so Gorilla Shaman is live against their artifact lands (Seat of the Synod, Vault of Whispers, Ancient Den) and noncreature artifacts like Cryogen Relic and Nihil Spellbomb, disrupting both their mana and their affinity cost reduction. Pyroblast is not a dead card here: it hits blue spells and permanents, and Baleful Strix, Utrom Monitor, Cryogen Relic and Thoughtcast are all blue. It stays in the sideboard to keep the swap small, but it is a reasonable option if you want more interaction with their blue cards. This matchup is still primarily a speed race since Affinity can also curve out quickly, so keep the core combo fully intact and make only a small, targeted swap.