They test with AIO. You need insanely low voltages for that. Many more 9900KS would easily reach 5.2 GHz with delid, direct die and custom water. Buildzoid stated that anything up top 1.45V is safe for everyday OC on this chips if you can cool them.
My friend has 9900K delided, direct die with custom EK loop (2x 360PE) for GPU and CPU. He dropped 12 degrees C with delid, and direct die. Sadly his CPU is a dud and won't go past 4.9 GHz no matter what.
Either way it costs a lot. You can go with binned CPU and cool it with AIO, or for the same money by stock CPU delid and go custom water.