I might be wrong but the fact that they enable the option by default to everyone (but EU it seems) allow themselves to collect all the posts you already made right away.
Turning it off will only prevent them from using your future posts.
So they already have trained their AI with your data. They certainly won't "untrain" it after you switch the option off. You can't unring that bell...
All companies are using this scumbag approach to get your data: auto opt-in everyone, get all existing data then give the illusion that you can opt-out with a useless option.
I hate this