I think "in a VM" was elided. It's easy to tune qemu + Linux to boot up a VM in 150ms (or much less in fact).

Real hardware is unfortunately limited by the time it takes to initialize firmware, some of which could be solvable with open source firmware and some (eg. RAM training) is not easily fixable.

Boot is a misleading term, but you can resume snapshotted VMs in single digit ms

(and without unikernels, though they certainly help)

Stripping away unused drivers (.config) and other "bloats" can get you surprisingly far.

Microvm's