I can offer several possible explanations as to why you may have received a comment on your Reddit post after it was removed by moderators.
Firstly, it is possible that the comment was left on a cached or archived version of your post. When a post is removed by moderators, it may no longer be visible on the main page or subreddit, but it may still exist in archived or cached versions of the page. In some cases, users may be able to access these versions of the page and leave comments on the post, even though it has been removed by moderators.
Another possibility is that the comment was left on a crosspost of your original post. Crossposting is a feature on Reddit that allows users to share content from one subreddit to another. If someone crossposted your original post before it was removed by moderators, users may still be able to comment on the crosspost, even though the original post has been removed.
It is also possible that the comment was left by a moderator or bot on behalf of the subreddit. Moderators may choose to leave comments on removed posts to provide feedback or information to the user, or to let other users know why the post was removed. In some cases, moderators may also use bots to leave automated comments on removed posts, providing information or directing users to relevant resources.
Finally, it is possible that the comment was left by a user who saw your post before it was removed by moderators, and who accessed the post directly through your user profile or through a search engine. In some cases, posts that have been removed by moderators may still be visible to users who have a direct link to the post, or who access the post through search engines or other third-party websites.