Pretty sure crossbreeds can happen as easy as any normal baby happens. I don't recall reading or hearing anywhere that it's difficult for races to crossbreed. Only that for some cultures(like Ishgard) it's looked down upon. There's even a Hyur guy in a SB side quest who outright mentions how he wants to marry a Miqo'te girl and have children with her.
That's why I'mma chalk it down to "It's SE" because it's been proven time and again that "rare" people who were otherwise never seen before, can be just..
made by SE and well they're real and all over the place now. Like Highlander/Roegadyn females, Miqo'te males, female Miqo'te children(males still don't exist yet

), Hyurezans, and the entire Au Ra race that flooded into Eorzea within a patch with no explanation until Stormblood where they also created Au Ra children.
Soooo... I think it's safe to go along with "It's SE"