“Uh, it was my Big Eater flaw.”

“But Luffy's Big Eater flaw doesn't do that.”

“I didn't say it was Luffy's Big Eater flaw.”

I could honestly see the justification for trying to use fish food to use the Big Eater flaw/bonus.

Especially with his low intelligence.

Sometimes you just have to make some asinine decision more no reason at all. It won't advance the narrative, and it won't even come up unless someone mentions it. But if they do, ooh boy, they're in for a surprise.