After doing some more digging, I'm positive that this is the `fqdn` of [shard nodes](https://github.com/Kabam/chef-mongodb/blob/3c0f4ef9357f2e5071ce302cafc080da37802f24/definitions/mongodb.rb#L194), not config servers. [It's used to set up the cluster](https://github.com/Kabam/chef-mongodb/blob/3c0f4ef9357f2e5071ce302cafc080da37802f24/libraries/mongodb.rb#L245) (see [`addShard` documentation](http://docs.mongodb.org/manual/reference/command/addShard/).

It probably still works because you only set `node['mongodb']['configserver_url']` on config server nodes, so it's `nil` on shard nodes.