To my understanding, the key part is that you are supposed to disclose any type of information that you are sharing with third-parties through back channels.
If you set a third-party tracking cookie on your site, then yes, the third-party can use the cookie to correlate users from different sites. But if you do what you just did and place a image that displays the IP, how can any third-party access this information? You have my IP and a request log, so what? Is there any way that another Lemmy instance can use this to identify me?
On the fediverse, every comment, every vote, every moderator action is completely public, and tied to the username.
And distribution/collection of public information is not what the GDPR is trying to regulate!