MongoDB - handling embedded documents and relations
You are not missing the point :)

The key is how many reads and how many writes do you have to do when you store/update/fetch various entities in your collections?

You are presumably constantly (daily?) creating new posts and frequently creating new comments, and very frequently querying posts.

Actions like updating authors' email are very rare compared to the above.  You want to optimize the performance of reads and writes that happen hundreds or thousands of times per  in your application, and worry less about performance of very infrequent actions.

Having said that, I would store the basic author information embedded in the post, but I would only have information there that must be displayed with every post.  If there are additional details about the author I want to keep in its own collection, I would expect that would be details that the user needs to click additional link to see (allowing for time to do another read).