It's literally been proven that they do. A guy here on Lemmy was a very common poster on some tech support subreddit. He used one of those account scrubbers and deleted his account. He went back to look a few weeks later and all his comments were back.

Because of GDPR, there should be a way to completely wipe your account

Yeah. At most they'd mark the comments as inactive, hide from the user accessible areas and maybe anonymize the user id. But they definitely have the username table and the data still in the system, 100%, just waiting for the right offer.

There are archives of all Reddit comments that are collected at the time of posting, all the deletion and scrubbing and whatnot people are doing months or years after the fact doesn't affect those.