How did NordVPN implement the kill switch in their iOS app?
Kill switches on iOS only work with the IKEv2 protocol.

IKEv2 provides resilience to the VPN connection. When the VPN client moves from one wireless hotspot to other, it automatically disconnects all internet activities when a VPN connection is lost and re-establishes the connection upon successful connectivity. 

Mobile users specifically can benefit from such a protocol. However, it is not supported on many platforms as it is fairly new to VPN services.

Apple has a very strict process for getting the entitlements to allow an app to control system wide network access. You need to contact them and answer all kinds of questions before you get the entitlements.