The latency to human user is what matters. If I get an answer in under 100ms as a user 99% of the time, I don’t care whether 100 or 100K times per day someone else hits that 1% worst case.
Also keep in mind how percentiles compound when you have more than one service involved in serving a customer request. For example, let's say it takes 5 internal requests to serve an external customer request and each of those services measures latency SLAs at the 99th percentile. The customer request may only finish inside the SLA 95% of the time (99%^5)
Agreed, which is why the most surprising part of this article for me was this phrase: "In our main data center serving non-China traffic".
The transit latency to the one main datacenter in the (non-China) world is a lot more significant than the server time here. (Over 50 ms just to cross the US; compare with their server-side latency numbers of 95%ile < 5 ms, 99%ile < 50 ms.) If they're serious about latency, their deployment is holding them back much more than their choice of programming language or algorithm.
Or maybe the client is not the user's phone, but some other service running within the same datacenter?