I think you need to make a special request to get that level of deletion that comes with gdpr. I'm not certain, I just remember other users specifically talking about how you need to send them an email so they have to comply.
I also wouldn't be surprised if their dataset is mostly stripped of user names to get around GDPR though I'm no expert.
All that to say I'd be very very surprised if they deleted comments in their dataset.
Very valid point of devaluating the user experience thought, especially when you take into account google searches. I'm sure they have already fallen off compared to a year ago where reddit would pop up half the time no matter what you searched.