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.)