They'll just delete your user ID field for whatever you posted - so it won't be associated with you any longer, at least on their systems, but they will keep the content for analysis.
That won't work. They also cannot retain data that could be aggregated to identify you as a person. Anonymising by removing an ID is not actually doing that, it's just theater. The GDPR has provisions for that. Bottom line is: if you start fudging things or working around it, you're going to get fined.
If that's true then (1) they're lying and (2) that's not covering it because just a few website visits later they could re-associate your old data with your 'clean' profile because it doesn't take all that many bits to de-anonymize a chunk of data.