The key thing to be aware of is that while only 4 name servers are listed any large DNS provider will have a vast number of nodes supporting DNS resolution using simple solutions such as load balancing and more advance systems such as Anycast. The result is that the 4 published addresses will be serviced by 100’s of nodes distributed across many data centres.
DNS information is also cached as hierarchical at your machine, your ISP, their DNS provider and so on.
The real issue for Tardigrade.io is not their choice of DNS provider, by the choices made by the node providers to the storj platform. As many node providers (including myself) do not have static IP addresses provided by their ISPs they depend on DDNS (Dynamic Domain Name System) services that maintain a map between the public IP address they are using and a DNS name. If one of these providers was to fail storj could lose access to a large number of nodes for a while.