With the block 3 design they have hardened all of the surfaces within the bottom skirt on the rocket so the blowback during reignition can’t melt anything. And the flanges on the turbopump exhaust exist to facilitate redesign and inspection of the system. Once they know the exact shape they need they can construct a single pipe with two flanges instead of three pipes with six. Flanges make bigger failure points than a solid pipe due to the seals.