He used the 240 FPS camera on an iphone to measure time from start of key movement to screen update several times and rounded to the nearest 10ms. He also used a 1000 FPS Sony camera to recheck the 40ms and under results.

He does mention that he includes the keyboard latency; other latency test results he found exclude that step.

I find it fascinating that you think no one would bother to read details about a nerdy subject on Hacker News. Why else are we here?

I find it fascinating that a nerd wouldn't open with a description of their methods, which is what nerd readers want to know right up front. It's buried on the last page.