Nope, he really means 24 minutes:
/**
* Set both the expiration time and the expiration variant.
*
* If the current time is below the variant, we don't update the session's expiration time. If it's
* greater than the variant, then we update the expiration time in the database. This prevents
* writing to the database on every page load for active sessions and only updates the expiration
* time if we're nearing when the session actually expires.
*
* By default, the expiration time is set to 30 minutes.
* By default, the expiration variant is set to 24 minutes.
*
* As a result, the session expiration time - at a maximum - will only be written to the database once
* every 24 minutes. After 30 minutes, the session will have been expired. No cookie will be sent by
* the browser, and the old session will be queued for deletion by the garbage collector.
*
https://github.com/ericmann/wp-session-manager/blob/master/includes/class-wp-session.php
