Do we really know that gpt-5-codex is a finetune of gpt-5(-thinking)? The article doesn't clearly say that, right?

I suspect that this is smaller than gpt-5 or at least a quantized version. Similar to what I suspect Opus 4.1 is. That would also explain why it's faster.

OpenAI say:

"Today, we’re releasing GPT‑5-Codex—a version of GPT‑5 further optimized for agentic coding in Codex."

So yeah, simplifying that to a "fine-tune" is likely incorrect. I just added a correction note about that to my article.