Belcher is a pure combo deck trying to assemble Goblin Charbelcher with an empty library, and their maindeck runs heavy countermagic like Counterspell, Force of Negation, and Disrupting Shoal, so Flusterstorm is valuable to blow out a counter war on a turn they try to protect their combo piece. Into the Flood Maw can bounce their Tameshi, Reality Architect or a key permanent to disrupt their engine and buy a turn. Veil of Summer does little here since Belcher plays almost no green-hateable removal targeting you and its hexproof clause is largely irrelevant against a deck that doesn't attack your permanents with blue/black targeted spells aimed at your creatures specifically, so it's a fine cut. This is a pure race where your own combo speed matters most, and Consign to Memory already in your maindeck is well-positioned against their many colorless artifacts and spells like Goblin Charbelcher and Lotus Bloom.