I believe they meant more so “cannot delete without having to access the mailbox”
See the comments from their related feature request.
So it seems they are already taking advantage of and using advanced off-boarding procedures in place currently with Admin Password Reset, and passwordless email-only account deletion.
As mentioned, I am personally very eager to see what additional features this will bring and how this can add to better management and deprovisioning of corporate user accounts.
One thing though as an aside, I wonder how much of this becomes moot seeing as a user can currently alter their login email. There are no enterprise policies to restrict users to a specific domain or prevent email changes.
Perhaps with the introduction of the domain verification this could be used to require a user of the Org has an associated email with that domain, or possibly just an enterprise policy to restrict users from changing their login email.