Why? In a universe in which Spike, Angel, Drusilla and Dalton are not vampires, Olaf isn't a troll, and Giles is the Shogun, why quibble about Ampata not being a mummy? She's still Peruvian. There aren't any mummies in Japanese mythology so keeping her as an Inca Mummy Girl wouldn't have fit the concept at all. Everyone in the whole thing is acting in roles rather different to their standard Buffyverse positions. That's the whole point. And there isn't a single OC (other than nameless peasants, guards, and ninja) in the whole thing (although there is one crossover character).
Wesley is a shipwrecked English mariner running a nightclub (The Bonze). Spike (under the name of Chopstick) is a wandering minstrel and ninja warrior. Drusilla is a ninja geisha kung-fu nun. Xander is a taikomochi (a male geisha specialising in clowning, rather like a court jester). And as for Kendra... I won't spoil it for anyone who hasn't read it. Why object to Ampata being a Peruvian Inca fleeing from the Spanish Inquisition?
Or could it be that the su_herald editor (who probably hadn't read the story anyway) didn't know who Ampata is? Surely not.