I had some doubts about the story as it didn't make a lot of sense, which was warranted as PCI-SIG now clarified that PCI Express 4.0 will not feature a slot power draw increase.
The goal is to increase the maximum power capabilities of PCI Express 4.0, but the motherboard power slot draw will remain at the current 75W limit:
PCI-SIG reached out to tell us that the power increase for PCI Express 4.0 will come from secondary connectors and not from the slot directly. They confirmed that we were initially told incorrect information. We have redacted a short passage from our original article that stated what were originally told, which is that the slot would provide at least 300W, and added clarification:
PCIe 3.0 max power capabilities: 75W from CEM + 225W from supplemental power connectors = 300W total PCIe 4.0 max power capabilities: TBD
New value “P” = 75W from CEM + (P-75)W from supplemental power connectors.