Anandtech's testing showed full load power consumption of 279.82 W (3960X) and 286.72 W (3970X). OTOH the "255W" Xeon W-3175X full load power consumption was 381.08W...

2990WX with PBO enabled went over 400W.

Overclocking chips does tend to cause power usage to spike, yes. That's not exactly a new thing, nor is the increase that surprising. A slight bump to voltage will cause power consumption to skyrocket. Heck, i9-7980XE's at 4.9ghz are pushing 1kW in consumption. Hence why intel's 28 core @ 5ghz demo used a 1700W capable cooler.

Goodness, I had no idea the power consumption of that Xeon was so high!

ah no Amd power numbers are usually on the money

How will they manage to package twice the cores at the same TDP? (3990X vs. 3970X, both apparently rated at 280W).

The power consumption of an individual core can vary greatly depending on its clock speed (and voltage, which can be reduced when running at a lower clock speed). Putting 64 cores into the same TDP as their 32-core chip is actually pretty easy. If you actively use all 64 cores, they'll be running at a lower clock speed than if you were only using 32 cores.

binning the Chiplets helps them a ton with this