IBM mainframes might but I have a very shallow understanding.

That's exactly the same as x86. Nested virtualization support is almost entirely in the hypervisor.

x86 doesn't have a firmware hypervisor that allows splitting into LPARs.

That's showing 3 layers of nesting: LPAR, hypervisor, nested hypervisor

	▲	bonzini 6 hours ago \| parent [-]
		On POWER, LPARs and the first level of hypervisor do not use any LPAR-specific nesting support in the processor. It's all handled by the firmware, not the hardware.