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So they lost 5% of performance, then applied FDO, a generic optimization technique, and gained back about that 5%. In other words, they could be ~5% ahead without the bug (at least 2%, if you consider there are some helpful interactions between FDO and retpoline mitigations).

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Not to my understanding. They were already applying FDO to all of their performance sensitive programs. They found that after FDO, the retpolines and the other work done to mitigate meltdown and spectre had negligible impact. This is because FDO replaces indirect branches with direct branch hints, so only in uncommon cases do you even have to execute the retpoline. That's my understanding, at least.

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