Given his dad is just as wacky but actually involved in ul'dahn politics that's blatantly false, and multiple characters in it appear in more serious story lines but remember you from hilde quests
Secondly, the localization team has significantly less power or clout about the narrative than Koji, however flippant (especially at something unofficial). Combine that with the fact the game has literally gone out of it's way to give an in universe explanation for quite literally every single game mechanic, including party finder as of 5.4-5.5, and I don't think anyone can argue in good faith that fantasias don't exist in universe.
At best it can be seen as an argument that ti undergo the massive restructuring if aether necessary to change species requires some significant changing in one's mentality.
Personally I just rp my Viera as always having been one, but I personally want eorzea to be a place that just goes 'you get the body you want if you're brave enough to make the changes'