I believe it’s a sync issue between instances. Eventually, his comment should update and show as deleted on the instance you’re viewing from. New material seems to sync immediately across most instances, but edits/deletions appear to be lower-priority and take longer. Some mod actions like thread removals also don’t appear to sync correctly right away.

At least, that’s been my experience, not sure how much truth there is to that. I’m using Kbin, which is also a bit weird with Lemmy content in the first place (for instance, Kbin apparently does not recognized locked threads from other instances, and will still let you comment, though they won’t appear on the original instance at all).