OH
I just remembered this
it makes a bit more sense in hindsight now that we know Nika is worshiped in Elbaph and that Vegapunk regularly visited Elbaph in secret
The more I think about it though, the less likely it seems that the nika fruit itself was Elbaph's national treasure. If it were you'd think at least one of the giant pirates would have seen Gear 5 and gone "Hang on, did you eat that fruit that's meant to be protected by our royal family?"