Index selection for mongodb official guide for storing comments

In this mongodb official guide on how to use mongodb in order to store discussion threads they suggest 3 strategies of which I chose the first one: storing each comment in its own document. In this strategy it is suggested to use full_slug for threaded discussions. full_slug is composed of a slug (which is an id of some resource in a CMS like Wordpress) and a date. This way sorting on full_slug gives results ordered hierarchically and chronologically.

In addition it's suggested to create 2 compound indices on discussion_id and posted fields as well as for discussion_id and full_slug.

Because the query to retrieve the comments sorts by full_slug which already includes time information why do we need to create an index for discussion_id and posted?

In addition, if we use full_slug doesn't this mean that the slug part must be guaranteed to be sequentially incremented? Because if slug is not strictly incremented then the sort will not be always correct.