Gender locked race?

[Unpronouncable]

Actually it does the opposite.. imo.

It says at post-puberty sexual dimorphism IS a thing.

Only when they're children do you look at their face and go "hmm not sure", although I doubt the lore writer meant it as "we can't tell, if we checked" just that physically until puberty they're more similar. Humans have this, to some extent as well. Men get MUCH bigger (usually), with all their muscle mass and stench coming along for the ride. Females growing other things and other changes. To us this isn't that big because we're used to it but just compared children to adults, we look massively different and it's all because of puberty.

The lore admits puberty is a thing and admits it creates distinctive differences. I doubt they'd write that line if the males were basically bosom-less females.

You write the line because there is a radical shift, like humans, except it's possible its even more radical because we're in a fantasy game.

So it was rather stated as a justification for having there only be one child-model for Viera as opposed to two (does this game even have non-Hyur child models?). I mean sure, though skipping a single child model would hardly make much of a difference imho.