Namely, the section on "Mapping knowledge webs" (around 3:33). It explains people can struggle to adopt concepts that are too far outside their current knowledge, but that it can still be possible for them to pick it up if they can build a path to it from their current knowledge. (This often presents itself as the "depth-first" learning style)
A lot of commenters here, talking about how monads are so simple and how not being able to learn them is just lazy, are misunderstanding how learning fundamentally works. You have to expand the knowledge graph in the direction of monads before it will be easy to pick up monads. You have to build a path. You cannot simply teach monads right off the bat and expect someone to intuitively "get it". People who intuitively "get it" are people who already have a path.