The GDPR doesn't care how the personal data was obtained, that's why Art 4 GDPR specifically calls it "processing", not "collecting". However, it does protect only personal data: https://gdpr-info.eu/art-4-gdpr/
If you sent an Email to a company you could argue the same, that they were not "collecting" it, you handed the data in yourself. It doesn't work that way.
That's why Art 15 GDPR allows you to demand a data takeout of everything they know about you, in case of Reddit that includes all your posts and comments.
And of course the "right to be forgotten" (Art 17 GDPR) covers any of that data as well, but again only if it is personal data.
Reddit now just says that with deleting your username they de-personalised the data, so it isn't covered by GDPR anymore. That's technically correct. But if you wrote a comment with personal data in the comment, e.g., "Hey, my name is Patrick Smithereens, I live on High Street 666 in 1312 Arblfarg" they would absolutely need to delete that.