>- no significant skin effect at 400Hz -> use speaker wire, lol.
What are you talking about? There's a very significant skin effect at 400Hz. Skin effect goes up with frequency. These datacenters use copper busbars, not cable, so skin effect is an important consideration.
At 100 000 A for a 100 MW data center at 1000 V, speaker wire is a joke.
You obviously need at least a dozen stands in parallel!!
Clearly skin effect scales with frequency but, 400 Hz is still low, only 2.5x lines frequency (the scale is by the root); so the skin depth is 3mm. 3mm on each side makes for a pretty hefty rectangular cross-section.