If you look at their username thing, you can see that they’re replying from Mastodon, not Lemmy. It’s some weirdness Mastodon does that Twitter also does where it tags the person you’re replying to and anyone tagged earlier in the thread you’re replying to by default, unless you delete the tag before you post. Most people don’t delete the automatic tags. So they probably didn’t do it on purpose and may not have even realized they did.

It’s how it works when you reply to a Lemmy comment from Mastodon. It will automatically tag the comment you are replying to and it’s parent.