Wait A Minute...

[Steelsheen]

It was done so the Matoran would work for them without questioning their reasoning for the forced labour

Yeah, but that doesn't explain why all the female Ga-matoran have a male "Toa of Water". If I remember correctly, the only Toa the Voya-Nui matoran had encounted were Jovan's team, who all had unusual powers and thus mixed genders. Perhaps that might explain it.

[ALVIS]

True, true. Tarduk's a smart person. I suppose the implication is just that he doesn't know everything about the elements of the MU.

[Master Inika]

True, true. Tarduk's a smart person. I suppose the implication is just that he doesn't know everything about the elements of the MU.

Well, this is an Alternate Tarduk, who declared himself "not a historian," while we know this to be false for the prime Tarduk. Simply put, Alternate Tarduk could be different in a variety of ways.

[Erebus]

If the Matoran in Voya Nui never knew what a Toa looked like, chances are they never saw Jovan and his team as Toa. It's easy to slip by the Matoran without being seen; the Toa Nuva pretty much did it when they returned to Metru Nui to gather up supplies and such before heading out to Karda Nui.

[Lewa0111 Nuva]

Regardless of whether or not the Matoran of Voya Nui/Southern Continent had ever seen Toa before, they had certainly never seen a Skakdi before, and probably had no idea that other beings existed who were not Matoran/Toa/Turaga or Rahi. Therefore, when they met a group of six elementally-colored beings, much taller than Matoran, wielding protodermis tools/weapons, and appearing from canisters, they could tell right away that the newcomers weren't Rahi (because of their intelligence and ability to speak the Matoran language) and were obviously too tall to be Matoran or Turaga. Therefore, what else would they think they could be except Toa? Not to mention that, according to Reidak at least, the Piraka did indeed introduce themselves as Toa of their respective elements, and Hakann and Avak had already staged that "rescue" of the Matoran earlier. And as soon as Vezok opened his mouth and spoke, as I recall, Dalu immediately figured out something was wrong, since she at least knew that Toa shouldn't be encouraging random violence and threats among the Matoran. Lewa0111 Nuva

[Harvali]

Another think to consider is that Matoran turn into what they think a Toa should look like. That is why Vakama and company look like Lihkan, and why Takanuva looks like the Toa Nuva. The Inika/Mahri were changed without Toa Stones, which is why they don't look like the Nuva. So perhaps the Voya-Nui Matoran thought that the Piraka believed that Toa should look brutal and strong, and so that is what they became as Toa.