That would be a confusion point technicality. If you realllllllly want to know...
The documentation is auto-generated based on docblock headers in the code. The core/me/GETindex endpoint has this
/**
* GET /core/me
* Get basic information about the authorized user
*
* @apimemberonly
* @return \IPS\Member
*/
That @apimemberonly tag says that the function can only be accessed as an authenticated user.
However, the return values are not being defined here explicitly and instead are being returned automatically from the \IPS\Member::apiOutput() method. That method does not know where it is being called from (for example if you fetch a topic, the topic's author information will be returned from this same method), so the return values defined for that method denote if they're available when the request comes from an API key or not.
/**
* Get output for API
*
* @param \IPS\Member|NULL $authorizedMember The member making the API request or NULL for API Key / client_credentials
* @param array|NULL $otherFields Array of additional fields to return (raw values)
* @return array
* @apiresponse int id ID number
* @apiresponse string name Username
* @apiresponse string title Member title
* @clientapiresponse string timezone Member timezone
* @apiresponse string formattedName Username with group formatting
* @apiresponse string ipAddress IP address used during registration
* @apiresponse \IPS\Member\Group primaryGroup Primary group
* @clientapiresponse [\IPS\Member\Group] secondaryGroups Secondary groups
* @clientapiresponse string email Email address
* @apiresponse datetime joined Registration date
* @clientapiresponse string registrationIpAddress IP address when registered
* @clientapiresponse int warningPoints Number of active warning points
* @apiresponse int reputationPoints Number of reputation points
* @apiresponse string photoUrl URL to photo (which will be the site's default if they haven't set one)
* @apiresponse bool photoUrlIsDefault Indicates if the value of photoUrl is the site's default
* @apiresponse string coverPhotoUrl URL to profile cover photo (will be blank if there isn't one)
* @apiresponse string|null profileUrl URL to profile
* @clientapiresponse bool validating Whether or not the validating flag is set on the member account
* @apiresponse int posts Number of content item submissions member has made
* @apiresponse datetime|null lastActivity Last activity date on the site.
* @clientapiresponse datetime|null lastVisit Last distinct visit date on the site.
* @clientapiresponse datetime|null lastPost Latest content submission date.
* @apiresponse int profileViews Number of times member's profile has been viewed
* @apiresponse string birthday Member birthday in MM/DD/YYYY format (or MM/DD format if no year has been supplied).
* @apiresponse [\IPS\core\ProfileFields\Api\FieldGroup] customFields Custom profile fields. For requests using an OAuth Access Token for a particular member, only fields the authorized user can view will be included
*/
The @clientapiresponse tag indicates the request must be made using client credentials or an API key, while the @apiresponse tag indicates the value is returned for any request. This is mainly because some data is more privileged than others and can't be returned for any old user calling the API.
In other words, the endpoint cannot be accessed via client credentials/API key, only as an authenticated user, and the values returned will only be those that an authenticated user are permitted to see. If a member object is returned elsewhere, however, the notes you are referring to would apply.