You haven't mentioned passwords.
He also hasn't mentioned that you should exercise regularly, eat vegetables, invest in index funds, and not get addicted to heroin. Why should there be anything about passwords in an article advising people on how to get accumulated cruft out of their email inboxes?
This will be brief.
Inbox zero is a valuable thing to maintain. Roughly promoted around the web as having an empty inbox.
An email inbox collects a few things: