It applies to comments and posts to the extent that they are attributed to you, about you and/or their content is specific enough to render you identifiable. In practical terms, this means that a data controller can in most cases keep the content of your post by anonymising it, which is why when you delete your account it shows "[deleted]" in place of your user name above your old posts. If the content of the deleted posts, even in aggregate, allows me to find out who you are, then you would have grounds to insist that the content be deleted (or sufficiently anonymised).

Source: I was a DPO for 5 years.

It applies to everything you post. The GDPR is supposed to level the power between you and organisations like Reddit. It give you the power to know what data they have on you, correct and delete it. If they don't let you have this power, you can file a complaint with you data protection office. It doesn't have to be a formal complaint. Just write them a mail with your problem.