I wonder how much of this is because the model has memorized the Fibonacci sequence. It is possible to have it just return the sequence in a single call, but that isn't really the point here. Instead this is more an exploration of how to agent-ify the model in the spirit of [1][2] via prompts that generate other prompts.
This reminds me a bit of how a CPU works, i.e., as a dumb loop that fetches and executes the next instruction, whatever it may be. Well in this case our "agent" is just a dumb python loop that fetches the next prompt (which is generated by the current prompt) whatever it may be... until it arrives at a prompt that doesn't lead to another prompt.
[1] A simple Python implementation of the ReAct pattern for LLMs. Simon Willison. https://til.simonwillison.net/llms/python-react-pattern [2] ReAct: Synergizing Reasoning and Acting in Language Models. Shunyu Yao et al. https://react-lm.github.io/