I guess the more valid question is how bad have it been for the Zulu if they have an archer in their capital while you are sitting outside with paratroopers.
Did they come back to life? Otherwise the units just doesn't make any kind of sense. AI would not have an archer if they have infantry.
But as noted that infantry can be from a project (bronze at Global Wargames gives one infantry unit), it could spawn if their cities are being burned to the ground or it can be from "Freedom-Self-Determination" (second tier pick; liberated city spawns six units and an arsenal) where someone brings them back or liberates them and it's spawns six units in their city.