Sadly this isn't new, its been known about for quite some time. Slightly irresponsible for that person to post and make a big noise about it without creating a github issue though. Ideally the lemmy devs will add code to automatically purge orphaned images. Hopefully this public noise around it will hurry this up.

I see this much like the extraneous data being stored in the database previously; low hanging fruit for devs to optimise and make hosting an instance less resource intensive. Yes there are possible legal risks for uploads that are not referenced on lemmy, but the reality is this could happen deep in a comment thread or a user profile anyway.