There’s a difference between a major worldbuilding secret that the lore is built off of, and the success of the line and story.
Why can’t it be both?
Furthermore, even if we define BIONICLE as within a sterile genre, the s h i f t from fantasy to sci-fi just testifies to the complexity of the worldbuilding. If the story pitch is “XYZ living on an island that’s secretly the face of a giant robot”, proper worldbuilding (if such a thing exists) would be to develop that as a religion that has evolved from the truth: the inhabitants, in the however many hundreds of thousands of years it has been since the Great Cataclysm - now worship the robot as a deity.
Additionally, it’s not as if the entire plot was set out by the writers from the beginning - they had no idea that it would be as successful as it was. As such, the stakes and scope evolved over time as the line progressed.
You just said that it’s a fantasy world turned sci fi, and then claim that it’s unrealistic?
It’s called a plot twist. One that makes you realize that you don’t have all the answers, that the world is much bigger than you thought it was, and that ties together the subtle mysteries previously introduced (the impenetrable layer of “rock” discussed by Onu-Matoran in MNOG, your aforementioned maori translations, the entire three virtues symbol, etc)