Miuana is a Land/Seafolk hybrid. Although she didn't have ENA at birth, it reactivated.
My guess is it was activated by those glittering effects that are showing the seaGod's power.

Now, we could always think that the parade with the statue and folklore and the story behind it were in fact describing what happened to Miuna: offering new females to the sea people. It's just that with time, nobody remembered exactly all of that works and the festivities became just that and it did not work anymore.
Miuna is a special case, being an Hybrid. If her female friend also activates an ENA, it could become a lot more fun.