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Not 100% true - a microcode-based CPU without microcode isn't able to execute anything, so CPUs will ship with an early version of the microcode that's then (as you say) updated during boot.

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So I believe this rules out a supply chain attack where someone buys a bunch of CPUs, infects them, repackages them and sells them as new.

If you can't do that, then this feels significantly less problematic.

They could potentially do that to motherboards, but they could do that anyway (physical access would give you as much access to flash as this vulnerability does). But yes, CPUs should be fine in that respect.

But that doesn't persist through reboots, does it?

Correct, the update has to be applied on every boot, but that doesn't mean there's no microcode in the CPU itself

And that information is clearly irrelevant and misleading in this context.

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