I'm sorry, but your interpretation here is completely wrong.
Facebook (and Google!) cares about performance variance because it has more of an impact on overall site performance than average performance does. Variable performance has huge impact on downstream systems, and you can quickly end up with cascading performance problems.
I think that quote is slightly misleading without more context. They prioritize optimizing variable-performing queries higher than others. They aren't going to be using slow queries on the Facebook home page.
Facebook (and Google!) cares about performance variance because it has more of an impact on overall site performance than average performance does. Variable performance has huge impact on downstream systems, and you can quickly end up with cascading performance problems.
I think that quote is slightly misleading without more context. They prioritize optimizing variable-performing queries higher than others. They aren't going to be using slow queries on the Facebook home page.