My guess is that they did not add in the client delay.
They added too much latency because they forgot to account for the latency they added in the client. You can see from their architecture diagram (Figure 4) that the latency measurement didn't include the client delay.
https://images.contentstack.io/v3/assets/bltad9188aa9a70543a...
The blog post states: "The existing network monitoring system measured the latency at the networking layer as shown by the green arrow."
But the blog post states: "a calculation error that only manifested in scenarios where the actual ping was significantly lower than the target latency".
Because realized ping is over by as much as original ping was under, so if original wasn't significantly under...