I certainly agree with Tim's points.
I don't know what Amazon has going on "under the hood" and can only speculate. In terms of OpenStack there is no one type of networking [1]. The majority of people (based on survey) use Neutron and OVS, but there are others that use the Nova plugin directly. Within Neutron alone there are dozens of drivers ranging from generic SDN control to vendor-specific integrations. There are even network interface vendors [2] with hardware specifically for OVS/OpenStack.
You have a great deal of flexibility when it comes to OpenStack and networking. For instance, I know of a major cloud provider that has a custom network management solution that conforms to the Neutron API, so as far as OpenStack components are concerned they are working directly with Neutron.
Cody
[1]
http://docs.openstack.org/security-guide/networking/architecture.html
[2]
https://www.netronome.com