That's fanon that is not based in the actual lore in the game. I don't remember a miqo'te with a hyur father from an event, but Koji already said that the hyur with a roegadyn dad is adopted. The only confirmed interracial children are the 4 half-Garleans and Hilda. 1 of those half-Garleans proved that "kids being the race of the mom" has no basis in fact since they look 100% Garlean except their third eye is black instead of white. The au ra story is a story from hundreds of years ago or more so we have no way of telling if that's actually true, especially since the story and the underwater au ra themselves, are based on a Japanese folktale.
Plus, if we're going by what Matsuno said on Twitter that FFXIV and FFXII viera are the same settings, then viera interracial relationships are considered abominations in their culture. If Rabanastre has had viera in it for a thousand years and no one has seen a male viera there, then they're obviously not mixing with the locals. While the races can interbreed, the writers of the game have stated several times that they generally just don't. Of the 6 confirmed interracial kids including the au ra, only half of those were the products of a consenting relationship.