>"Like the author, I really cannot wrap my head around this. While I understand that duplicating documents across collections may make querying faster, what about when you want to change the document? "

That's a trade-off you have to analyze to see if it is worth it. If that data is not maintained often then is OK to have duplicated data. One thing you have to realize is that there is no a silver bullet in the CAP theorem and you always would give up something.

The other thing to realize is that not everybody needs a distributed database and most problems will live happily with an old-fashioned relational database.