On iOS a paid download is a transaction that is 100% managed between apple and the user, and as a developer you receive daily aggregate statistics. You don't receive any user identifiable information that you could use to limit accounts. There is no way to supply serial codes as part of a purchase. There is no way for them to create a single account as a part of the purchase.
The only thing they could have done is to change their game to a free download and make multiplayer an in app purchase. This would, of course, require that they also make something that is playable outside of multiplayer (since apple won't allow you to ship something that is non functional in the default state). Also, the server-side verification of IAP receipts can fail, which if they do strong checks can block potentially valid users from playing, through no fault of either the player of the developer. (notably, there have been cases of apple's verification server going down or incorrectly reporting receipts as invalid)
So please, before you eviscerate the devs, please realize they were trying to do the best they could for the consumer, and as a result got thoroughly screwed over. I wish them the best and hope they recover.