We may disclose personal information to: ... third parties, including agents or sub-contractors, who assist us in providing information, products, services or direct marketing to you.
There's no marketing in iMessage, certainly not based on the things I type into iMessage.
Also, to be clear, "marketing" is advertising. You say "ad-free" but then you give yourself the right to advertise.
There's also nothing I can find about end-to-end encryption, which is one of the best features of iMessage. I almost exclusively use iMessage because I know Apple isn't spying on me. When friends message me on Instagram, I reply in iMessage.