They can update their privacy policy at any time so it wouldn't have mattered if they added it after you made your account.

Do you have some expectation that when you post your content to some 3P site that you somehow continue to exercise control over it (other than rights under the GDPR)? What basis do you have for this belief?

Functionally, it doesn't matter anyway. These licensing schemes only serve the owners of services large enough to legally badger other moneyed entities into retrospective payments. Individual users have no agency over their submitted content, and nobody in charge of these companies even gives a second thought to keeping it that way. As I've said many times, nobody in this space gives a shit about anything except how they look to investors and potential users-- least of all the people that make the 'content' these machines 'learn'.