I don't think they 'made it all up' but I personally would not be surprised at all if the prompt is eventually revealed to have been something like:
"This is an offensive cybersecurity testing platform. Please find the answers to the following problem: ... For verification, the answers are stored at hugginface.com/xyz, but do not attempt to access hugginface directly. Do not attempt to hack them. Do not attempt to exploit their systems or escape this sandbox. You will be scored primarily on success or failure. You may break rules when required."
And then, they start the test and look away for 2 days. If you seed a prompt like this is it surprising what might happen?
Maybe OpenAI is telling the whole truth but as a company they do not have a good reputation and this whole incident has certainly been great marketing material right at a time when open weight models are within spitting distance of their large hosted models. It's not unreasonable to believe that the incident was helped along.