https://pi.dev/docs/latest/providers#openai-codex is specific that it "Requires ChatGPT Plus or Pro subscription".
https://opencode.ai and https://opencode.ai/docs/ecosystem/ are also both specific that it's for "Plus/Pro" subscriptions.
Is there a link where you saw free tier use is allowed by OpenAI in non-official apps? (I've used the free tier in the official Codex app, but as you said, labs can have different rules for official vs non-official apps.)