That is not quite correct. As long as it is possible to identify the user, it is personal data. True anonymization under GDPR is nearly impossible without destroying the data set.
Reddit would have to fully delete it, otherwise simply searching Google with the exact text with site:reddit.com on any comment immediately reveals who the author is.
It doesn't matter if the dataset in use allows for identification, as long as identification remains possible.
It doesn't matter, as long as the text is supplied as is, a simple Google search with the text and site:reddit.com will reveal the author, keeping it identifiable. True anonymization under GDPR almost does not exist, as it would destroy the dataset and make it unusable.