You're all good, no worries.
Antik is one of the admins of lemmy.world. He holds administrative authority over the entire forum rather than being specifically involved as a moderator here. He could have removed the comment himself, but he chose to request you change it instead (which I would have also done, had he not already handled it).
As a moderator of this community, I'd like to express our shared sentiment. The use of the term "retard" as an insult is something we discourage. Its appropriateness in language has evolved over time, and it's increasingly considered inappropriate due to its derogatory associations, regardless of its original meaning. Language is subject to change, and words that were once commonplace are regularly perceived as offensive now. Regardless of personal views on the word, if the admins have deemed it unsuitable for use on this platform, it's something that we will also enforce.
There is an abundance of other words that effectively convey your message.
He's been banned and (most) of the comments removed.