While 129.134.30.0/23 (subnet where and a and b nameservers reside) has indeed been withdrawn (according to FB postmortem by DNS automation tooling, 129.134.0.0/17 that is the shorter prefix (perhaps summary at the edge) was still present, however,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.