I noticed that the README states:

Calls to the Google Ads API will fail if the user that generated the OAuth2 credentials (Access Token and Refresh Token) doesn't have direct access to the Google Ads account to which the calls are being directed. It's not enough for the user to have access to a MCC above this account and being able to access the account through the interface, it's required that the user has permissions on the account itself.

However, the Google Ads API client library for Java supports auth via manager access by specifying login-customer-id as described in https://developers.google.com/google-ads/api/docs/client-libs/java/config-file and https://developers.google.com/google-ads/api/docs/concepts/call-structure#cid.