While 129.134.30.0/23 (subnet where and a and b nameservers reside) has indeed been withdrawn (according to FB postmortem by DNS automation tooling, 129.134.0.0/17 that is the shorter prefix (perhaps summary at the edge) was still present, however, didn't have longer prefixes (e.g 129.134.30.0/24 and 129.134.31.0/24 we normally see anycasted externally) internally.
In other words - routing towards FB DNS subnet (I haven't looked into 185.89.218.0/23 which is where 2 other authoritative nameservers reside) still worked up to the FB border, the traffic was dropped (routed to Null) by FB edge, since it didn't have more specifics internally.
This, combined with TTL of 60 seconds led to almost immediate global DNS failure and all other stuff you have been reading about.
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.
While 129.134.30.0/23 (subnet where and a and b nameservers reside) has indeed been withdrawn (according to FB postmortem by DNS automation tooling, 129.134.0.0/17 that is the shorter prefix (perhaps summary at the edge) was still present, however, didn't have longer prefixes (e.g 129.134.30.0/24 and 129.134.31.0/24 we normally see anycasted externally) internally. In other words - routing towards FB DNS subnet (I haven't looked into 185.89.218.0/23 which is where 2 other authoritative nameservers reside) still worked up to the FB border, the traffic was dropped (routed to Null) by FB edge, since it didn't have more specifics internally.
This, combined with TTL of 60 seconds led to almost immediate global DNS failure and all other stuff you have been reading about.