Has nothing to do with modern. ARM CPUs typically do not have turbo mode, which generally requires addition of another voltage supply rail. As i said, i expect that most of the logic has single supply, then another for SRAMs, then another for analog and then potrentially a fourth for some peripherals.Modern CPUs aren't restricted to one digital domain internally though I'm less familiar with ARM, I'd be surprised if they were different in this regard given the SOC design.
The 105 W TDP isn't based on hardware measurements, it's based on RTL simulations with an unknown configuration for the SOC portion.
Besides gate level or RTL level power simulation are very precise these days - you dont need power measurements in order to make an accurate prediction. ARM is very precise that the 105W TDP estimate refers to a 64 core hyperscale reference design running at 2.6GHz.
Not sure what issues you have with both ARMs simulations and the scaling from Andrei, both arriving at the same conclusion. It is not necessarily 80W like the lower bound of Andrei's estimate but certainly below 105W.