Saturn knew Luffy was Dragon's son,he knew he eat the nika fruit or both.
I think that what would make more sense is that the World Government erased "Nika" from the World's existence.

They erased the Buccaneer race because they were the ones who were spreading Nika.

The crime of the Buccaneers? Having the desire that one day, a warrior of liberation appears to free them (Nika/JoyBoy).

The World Government thought that by erasing Nika's existence in the world, the Nika fruit would never awaken, so it was a legend even for them.

That's why the fruit was renamed into "rubber fruit", if nobody wishes for Nika, the fruit can't awaken, simple as it is.

However, people kept desiring for a "warrior of liberation" to come to them (especially in Wano) so Nika/JoyBoy came back.

That's why the Gorosei were shocked, they thought that it was "impossible" for Nika to come back since it's a legend even for them.



However, as Vegapunk said, as people keep wishing for Nika, his existence will never be erased.

 
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