I think at high speeds you do actually end up optimizing force per press and time spent on a key out of necessity. It's like cycling, where they say "it never gets easier, you just get faster", or in other words, they can do a lot more with the same amount of effort.
I actually keep my nails pretty short (they're only about 1 or 2mm past the furthest-out part of my fingertip) for typing reasons!
My speed gets a lot worse if I let them grow out, so I like to file them down regularly (instead of clipping them occasionally) to keep them at a length I'm used to.
They increase stiffness. You hit the keys with the hard nail instead of the squishy flesh of your fingers, which reduces jitter and improves timing accuracy.
(I'm a fast typer, but not as fast as that one, also with long-ish --- around 0.050" --- fingernails.)
Tried that and got 109 WPM with 100% accuracy on my Macbook Pro (non-butterfly) keyboard. I usually score around the 80s with regular sentences and punctuation on other sites though, so subtract 30% from the WPM results on this side to estimate a regular WPM.
Here's what I personally use to test my keyboard setups: monkeytype.com.
My best is ~95 WPM and 100% accuracy.