They can’t unless the parties agree that they can. The sneaky part is the “by continuing to use our services, you agree to the new terms” part, which is standard practice. You’d have to terminate your account before the new terms come into effect, then take them to court to make sure they didn’t keep your data around and use it to train their AI anyway because they “didn’t notice” that that particular content belonged to someone who didn’t accept the new terms.