The Configure Atlas API Access section of the documentation contains the following instructions:

Enable the Atlas API

For each group that will use the Atlas API, do the following:

1. Click Settings, then Group Settings.
2. Toggle the Enable Public API field to On.

However, the Settings : Group Settings page does not contain the Enable Public API option. My understanding is that the public API is enabled by default and that access is controlled via the API whitelist/keys settings. If this is the case, the documentation should likely be updated to accurately reflect this.