Former humans influenced by a fiendish taint, tieflings are the evidence of mortals consorting with evil extraplanar powers.
There is no reason that such a thing could not happen in Escarnum; mortals still seek power from sources outside their world, and evil entities of all sorts still seek to corrupt and control mortals and their affairs. In fact, many demons and devils are particularly eager to exert their influence on the material plane, in hopes that they will eventually attract a cult following.
However, the planes of Escarnum are more abstract than those of other settings, and their borders more difficult to cross; a creature like a tiefling would be all but unique amongst the “ordinary” races. Most likely, a tiefling would not be born, but would be created by an infernal creature seeking to empower one of its followers—or the unfortunate child of such a follower.
A tiefling may also represent an actual devil or demon, who has taken on material form to achieve some goal in the mortal world. Infernal creatures often tempt mortals into such summoning them with clever bargains, which enables them to share the summoner’s life force until the bargain is fulfilled. However, most dislike long stays in the mortal world—they find that “being meat” has a way of leading to decidedly mortal sympathies.