This is true, but I can't imagine Google/Amazon/Microsoft are using Cisco/Juniper routers at every level of their network.
Hosts play a big part of SDN in that they support the dvswitches along with guest VMs. Not everything is a Cisco/Juniper. Switching hardware is still common in TOR and egress.
VLAN's and Virtual Appliances in the same environment as the guest machines to facilitate routing should allow for scale without costing these virtualization providers too much.