ChatGPT is running a special model but it's also available through the API: https://platform.openai.com/docs/models/chatgpt-4o-latest
The system prompt may still make a difference though.
I could rant for quite a while about how OpenAI and Anthropic manage their apps vs their APIs. It's really quite strange that they both landed on the solution of non-public APIs that perform differently than their public APIs.
https://imgur.com/cO7eFNt
o3 Chat is also similarly wrong, saying {4}.
Also I think the authors used the API, and maybe there are differences between the API and chatgpt.com behavior...