This caveat is mentioned at location at 3:10 into video.
The caveat in question is ‘Any of the indexed fields are embedded documents’. The video implies that this index can’t be used to serve a covered query.
However, mongo DB docs say: “Changed in version 3.6: An index can cover a query on fields within embedded documents.” (https://docs.mongodb.com/manual/core/query-optimization/#covered-query).
I suggest this be added to the notes associated with this video.