Maybe they accidentally commented with their porn alt? Seems like the comment was deleted.

I literally just joined about an hour ago, so take my comment with a grain of salt, but maybe you can manually track the communities you sub to on your current instance in case that happens? I did some reading and it seems possible to set up something like an RSS feed for Lemmy communities, so you would still be able to view activity without an account (if you instance goes down), and you have something documenting the communities you are interested in. Sorry if this is not useful, just trying to do my part and be more active here.