Seems pretty clear to me. DL isn't going to stop you from buying rev tickets for INF. But they aren't going to go out of their way to OP-UP one into the J cabin either. That's likely the right balance they are looking for from a product experience perspective.

I've traveled in J several times with INFs in the cabin. For the most part it was a non-event. But I can see it going bad real fast in certain circumstances.