They only need to delete personally identifying data. That means they could probably just remove the username from your post, but leave the post text there attributed to "anon".
yes, and if they cannot, they are violating the GDPR and have to pay fines.
This is why datasets are rarely sold as massive CSV's in the modern world - instead companies with data sell access to API's, with the expectation you'll query for what you need in real time.