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.