He probably means its a degenerate case of the list monad. Which is a useful way to look at it: a Maybe value is like a list of length at most 1. You can also

He probably means its a degenerate case of the list monad. Which is a useful way to look at it: a Maybe value is like a list of length at most 1. You can also look at it as a degenerate case of Either. (That is, Maybe a ~ Either () a.) This is "degenerate" in the mathematical sense, much the same way you can think of a point as a degenerate circle.