That is insane. Sucks that the Linux patches, at least, are rolling out for everyone, so AMD users will still see the performance hit even though they're apparently not vulnerable to the bug.
The Register thinks that AMD kind of spilled the beans on the nature of the vulnerability, and has seen proof-of-concept code that appears to confirm this:
It appears, from what AMD software engineer Tom Lendacky was suggesting above, that Intel's CPUs speculatively execute code potentially without performing security checks. It seems it may be possible to craft software in such a way that the processor starts executing an instruction that would normally be blocked – such as reading kernel memory from user mode – and completes that instruction before the privilege level check occurs.