This seems to cover everything about getting to "inbox zero" except the nontrivial bits of actually getting to "inbox zero".
That is: I bet most people with overflowing inboxes have lots of things in those inboxes that they can neither classify immediately as "no more to do" nor resolve in a few minutes. And what stops those people getting their inboxes down to zero is (1) all the work required to deal with those things, and (2) the psychological discomfort caused by thinking about #1. And nothing in here says anything about how to deal with that situation.
And nothing in here says anything about how to deal with that situation.
I read the advice as:
If you still have unresolved emails from 2015 in your inbox then keeping emails in your inbox isn't causing them to get resolved. Accept that, get a clean slate, and move on.
Make a folder called "old inbox" and put all your old emails there. Now you have an empty inbox! The costs of putting your old emails out of sight are less than the benefits of keeping an empty inbox going forward.
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: