I don't think Katakuri even thought is was possible that someone would eat there way out .
Don't think he underestimated Luffy but had no idea some one would\could even do that .
He won't know that luffy is moster also when it comes to eating lol.
@Kaido:
He may not have underestimated Luffy's fighting skills but he definitely underestimated his appetite lol
To be more precise, he left him there to slowly suffocate to death as he went to go and eat; neither could Luffy move. He had no idea that Luffy has that kind of appetite to the point where he'd eat his way out. So I give it a pass tbh
I guess Luffy being probably the most monster entity in One Piece regarding food solves the mystery :P