Re: Redmond's Windows Cloud?
"Isn't that Windows Cloud limited by the scalability of Windows Server?"
That's a pretty high limit. See https://technet.microsoft.com/en-us/windows-server-docs/compute/hyper-v/plan/plan-for-hyper-v-scalability-in-windows-server-2016
"Or is Microsoft secretly running Linux on their Cloud
They certainly run Linux on their cloud (~30% of VMs). If you mean does their cloud run on Linux then that likely would limit scalability. There are plenty of benchmarks that show that Hyper-V outperforms say KVM.
"with their Linux switches"
The SAI API is, in principle, cross-platform, defined for both Windows and Linux, but much of the vendor switch hardware is supported best, or even exclusively, in Linux. Hence why Microsoft used Linux for network hardware.