And the way half elves and half orcs were?
Yes, [tieflings] are mechanically a unique race option. That's part of the point I was making. They are, in the lore of D&D though, hybrids between the lower plane and elves, humans, orcs etc. Very similar to Half Orcs and Half Elves in being descended from two or more species. Point being that they got to retain their mechanical uniqueness while Half Elves and Half Orcs were stripped of it. Tiefling players don't get 'just play a human and describe your character as having horns and a tail
I’m not talking mechanically. I’m taking lore. In the One DND play test material, ardlings and tieflings are not described as mixed races. They are presented as their own races. Not necessarily descended from of mix of two different species.
While the description of the races doesn’t preclude this, it seems that the way WotC is heading is that most tieflings are the offspring of two tiefling parents and ardlings are the offspring of two ardling parents.
That is different from somebody who has one human parent and one elf parent or is a mixed dwarf-gnome hybrid.