That documentation doesn't really apply to the question I was asking.
Regardless, the answer, for whatever reason, was to add the calendars to the Outlook Mobile App in my account. Once that happens, the shared calendars appear in the call GET https://graph.microsoft.com/v1.0/me/calendars
Adding them to Outlook proper or OWA doesn't add them, but for some reason the Outlook Mobile App seems to reveal them to Graph. If anyone understands this behavior and why it is so, please let me know!