Maybe I am missing something from the docs of your link, but I unfortunately don't think it actually states anything regarding allowing users to connect and use their Codex quota in third party apps.
From TFA: “OAuth authentication (used with Free, Pro, and Max plans) is intended exclusively for Claude Code and Claude.ai. Using OAuth tokens obtained through Claude Free, Pro, or Max accounts in any other product, tool, or service — including the Agent SDK — is not permitted and constitutes a violation of the Consumer Terms of Service.”
https://x.com/thdxr/status/2013010664776683644
I can't find anything official from OpenAI, but they have worked with the OpenCode people to support using your ChatGPT subscription in OpenCode.