You are right on the mark here. I've worried over this and come up with an hypothesis or two. First, Carsini cut off the air to suffocate his brother. Second, he did it to control the temperature of the corpse, a theme in many an episode. Since Carsini threw the body in the (understandably cold)ocean, temperature control was meaningless, so then he was trying to suffocate his brother. That would look like the brother lost oxygen during his dive. If you look at the wine cellar, you might notice that the volume of air inside would more than sustain an unconscious and unmoving person. Columbo, because of time constraints, uses a kind of shorthand for murders. The victim is in a sealed room, he suffocates. The victim is shot, the bullet went through his heart. The victim is poisoned, it was phenobarbital.
So, Carsini's brother was left in a very large cave, and he suffocated in a huge volume of air. The underground wine storage room, originally designed to control temperatures prior to modern air conditioning, being impervious to temperature changes, would not allow the wines to be corrupted in the short time allowed in this episode.

In watching Columbo, we cannot be less observent than he is. As fans, we are our own Columbos, and Columbo is our main suspect.

I love that!