Obviously you're not wrong about fire being a major blocking point for sea creatures[1] - hard to make metals without fire. But possibly mermaids could "cook" their food via fermentation, e.g. putting food in a sealed swim bladder, then burying it in the sand. This could fulfill the same brain calorie requirements.

[1] Of course, maybe in Andromeda some sea-hackers are reading Hacker Sea-News and arguing land-based animals could never develop civilization because something-something.