Why does Comfrey Break Down Easily?
Erin, the main the problem with the question is apparently this part "One of its features is that the leaves break down very quickly, either in water or in a container closed from UV light." You also mention the compound comfrey and wonder if this is responsible for a quick decomposition? I don't think so.

The key question is that Comfrey plant decomposes quickly, as you observed, but compared to which plants? I don't think it is unique that it decomposes rather quickly in soil, water etc. Don't other plants like spinach leaves decompose or spoil quickly? You can easily eliminate UV because UV cannot penetrate compost. It must be bacterial / fungal action.

You can use Google Scholar to fetch scientific papers only and one of the scientific papers says "Comfrey on a dry weight basis is very high in ash, averaging 18 percent at Rosemount and ranging from 13 to 42 percent in other parts of the world (4). A partial
elemental composition of comfrey forage is reported in Table 4. The forage is particularly high in potassium and is higher than many other forage crops in calcium,
phosphorus, iron, and copper. Carbon, hydrogen, oxygen, nitrogen, and sulfur are the major non-mineral elements. The combined high nitrogen and high mineral concentrations make comfrey forage an unusually good material for composting, mulching, and organic fertilization of crops"

From Comfrey: A Controversial Crop https://conservancy.umn.edu/handle/11299/141442

The high nitrogen content could be reason for its quick decomposition by bacteria or fungi, however it is hard to assign a single compound "comfrey" as being responsible for it.