The promise about not using your messages for ad targeting only covers "personal" chats. Messages you send to business accounts, anything you type to Meta AI, and posts in public-style features like channels are specifically listed as not personal, so that content can be used to shape the ads you see. This is disclosed clearly rather than hidden, but it means a chat with a store or a question to Meta AI is treated very differently from a chat with a friend.
“Some types of messages that aren't personal include messages to and from business accounts, messages exchanged with Meta when using products like Meta AI, and messages in features designed to be more public, like channels.”
What you can do — Treat chats with business accounts and with Meta AI as non-private: avoid typing health, financial, or other sensitive details there. Use regular personal chats for anything you don't want feeding ad targeting.