Well technically you're already running Arch with a Debian kernel before the reboot. You could install and start services, the guide does so for sshd. Now -- taking off my pedant hat -- clearly you wouldn't want to rely on this chimera, and rather let it settle into one shape through a reboot.

My success rate with such projects is sufficiently low that I only attempt it when I have no other option. And that's for Debian -> Debian bootstrapping. Most of the time I manage to fumble a crucial parameter and the boot fails. I wouldn't even think of doing this remotely except as a challenge.

But that's not to mean you shouldn't try it for the fun of it!