That particular subnet has a covering prefix, but I don't think the other two DNS subnets do, and I had checked on the WhatsApp authoritative subnets, because I have greater affinity for WhatsApp. The WhatsApp subnets don't usually have a covering prefix (and I did check a looking glass during the outage and there were no announcements visible at least at that point).
For those with a covering prefix, the diagnosis is a little bit different as you said, traffic would still flow to whichever FB PoPs advertise the covering prefix, but then it loops in FB, because the PoP doesn't know where to send it, since nowhere was advertising the specific /24. As opposed to the addresses with zero announcements, where the traffic doesn't make it to FB, but gets dropped somewhere else.
For those with a covering prefix, the diagnosis is a little bit different as you said, traffic would still flow to whichever FB PoPs advertise the covering prefix, but then it loops in FB, because the PoP doesn't know where to send it, since nowhere was advertising the specific /24. As opposed to the addresses with zero announcements, where the traffic doesn't make it to FB, but gets dropped somewhere else.