Tao says that isn't the case for all of it and that on massive collaborative projects he's done many nonmathemeticians did sections of them. He says someone who understands it well needs to do the initial proof sketch and key parts but that lots of parts of the proof can be worked on by nonmathemeticians.
If Tao says he's interested in something being coded in lean, there are literal teams of people who will throw themselves at him. Those projects are very well organized from the top down by people who know what they're doing, it's no surprise that they are able to create some space for people who don't understand the whole scope.
This is also the case for other top-profile mathematicians like Peter Scholze. Good luck to someone who wants to put chatgpt answers to random hypotheticals into lean to see if they're right, I don't think they'll have so easy a time of it.
