Hello Chris. I tried following the tutorial's instructions and did not encounter any issues following the instructions. I was able to reproduce the invalid_client error by supplying an incorrect client id or client secret.
You say you had errors uploading files even before un-commenting the "OAuthConfig" line. The uploading and viewing of files worked properly for me without authentication in place.
You might want to double-check that the bucket and client ID you are using belong to the same GCP project that you are logged in to on your machine or deploying to.