Despite the lower part of the collision on the composter being lower than a cauldron, items can't go through it to a hopper.

Steps to reproduce:

• Place a composter over a hopper
• Throw any item inside the composter
• Profit

Picture 1 and 2: Images serve as a comparison between a Cauldron and a Composter. In Picture 2 we can see the space between the player feet and the hopper being higher than in the composter, as seen in Picture 1.
Picture 3: Item inside the composter not going inside the hopper.