My observation at the moment is that we haven't seen ML formalize a cutting-edge math paper and that it did in fact take a lot of experience to pull it off so quickly, experience that's not yet encoded in ML models. Maybe one day.
Something that I didn't mention is that Terry Tao is perhaps the most intelligent, articulate, and conscientious person I have ever interacted with. I found it very impressive how quickly he absorbed the Lean language, what goes into formalization, and how to direct a formalization project. He could have done this whole thing on his own I am sure. No amount of modern ML can replace him at the helm. However, he is such an excellent communicator that he could have probably gotten well-above-average results from an LLM. My understanding is that he used tools like ChatGPT to learn Lean and formalization, and my experience is that what you get from these tools is proportional to the quality of what you put into them.