The Tahu GWP set was released with no promise of other Toa existing. It was simply a callback to the original Bionicle. Many people have made their own fan designs for the other Toa, including TTV, who hosted a building contest a while ago with the fan-made Toa as prizes.
Because the designers behind that set are fans of Bionicle and wanted to give people the option to make Onua if they so choose. This was likely the only set LEGO would allow them to sneak it in.