It’s quite easy to anycast an IP to multiple physical datacenters. Their DC nomenclature is probably a relic from when they really had a single datacenter for each region.
On the other hand, it wouldn’t shock me if they had no site redundancy. It’s a free service, it’s not like there’s SLA agreements you paid for on signup.
The article says each user is associated with a single DC, and if that DC is down the user has no service. You kind of have to conclude from that the users data is not replicated across sites or there is some other architectural issue causing this. And they do have paid tiers.