Looks like they're doing allnoconfig so there's not much to build.

With make allnoconfig it also takes <30s on a recent-ish Intel CPU (12th gen; i7 12700). As low as 20s if it's cached and with make -j20.

1. modern computers are insanely fast

2. they compiled a version of the kernel with as many configurable options disabled as possible

3. modern computers are insanely fast