False.

There is a 3-level hierarchy:

System prompt > Developer prompt > User chat

You provide that middle level.

Ther may be actually no way to ever know. A baked in bias could be well hidden at many levels. There is no auditing of any statements or products from any vendor. It may not be possible.

Exactly my point but that person seemed to have insider info or a source we all missed.

It's common knowledge. See, e.g.:

https://lunary.ai/blog/openai-developer-role