Well definitions in shounen don't really mean anything.After all, since when did ninja mean wear orange jump suits and announce you're there? Or Shinigami/Death God/Soul Reaper/Grim Reapers being spiritually human who fight using weapons that're alive. Seems Zombie is the term being used in the manga in this case. I believe it's because they have no mind of their own. They're being controlled. They should use the word 'Puppet' instead of zombie.