Nick linking is the attempt to reveal multiple nicks as belonging to a common person who has not expressed a desire for that to be known. The warning here was misstated. The post did not contain nick linking, but rather "outing", which is the act of attempting to reveal the true identity behind a particular nick. You were warned about this in regards to this same person once before and, if I recall correctly, the link that was edited out last time was to the very same article.