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.
And most importantly and TFA mentions it several times: stripping unused drivers (and even the ability to load drivers/modules) and bloat brings very real security benefits.
I know you were responding about the boot times but that's just the icing on the cake.
(and without unikernels, though they certainly help)