The CPUs are x64 but the architecture is not that of a PC, there is no BIOS, etc. You couldn't boot Windows or Linux on the bare metal. The hardware, firmware and hypervisor are custom built for control, safety and observability. On top of that, the application OS all run on VMs which _do_ have a (virtual) PC architecture.
To me as a casual outside observer, the fact that they're using hardware virtualization at the top of the stack, after bcantrill gave so many talks about running containers on bare metal, is the most disappointing part. They could have had unbroken control and observability from the bottom of the stack all the way to the top. They got so close!
It's possible that the hypervisor can reserve you a full CPU or full cores for the guest OS to work with, so you still get most of that bare metal goodness.
