Reddit owns/uses multiple domains and the multiple domains can certainly collaborate to compromise your anonymity (or at least pseudonymity) even with third-party cookie blocking enabled. For instance they use redd.it for url shortening like this url. You could clear all your cookies for *.reddit.com and perhaps hide that you are the same user they previously banned for a while, but if redd.it has ever set any cookies that you haven’t cleared, the first time you visit a redd.it it could reveal your identity to Reddit. They probably own quite a number of other domains that might similarly reveal your identity to Reddit, and there are most likely third party companies that own other domains that will collaborate with Reddit in some way or another that will reveal your identity to Reddit.