Meh, how is that any different than "Elves hate orcs because the elven god put out the orc god's eye"?
Mostly because of where the gameplay weight in that sentence lies.
In "Tieflings are fiendish humans because of their ancient pacts with devils," one assumes that gameplay elements will focus on those ancient pacts with devils. Adventures will go to the ruins of their empire, devils will come seeking PC tieflings, tieflings will be inclined to manipulate and tempt, feats will make the tiefling more "devilish," etc.
In "Elves hate orcs because the elven god put out the orc god's eye," the gameplay elements more revolve around the fact that elves hate orcs. Adventures will have elves and orcs fighting each other, orcs will favor PC elves, orcs will be inclined to deforest and burn things, feats will make elves hurt orcs better, etc.
The former is very campaign specific, limiting to homebrewers, and less relevant to campaign settings where tieflings are, say, untouchable outcastes cursed by the good gods, or where they are fiendish humans because of their genetic heritage. The latter is much more broad -- elves could hate orcs for any number of reasons, and orcs would still burn things, elves would still hunt them, etc. The equivalent would be, say, designing elves to be good at blinding people in combat. That would reference the world-specific myth, and be less useful to homebrewers.
IF 4e's mechanics focus on "because," then they will be limiting. If the focus is broader, on "what they are," leaving the "because" open to individual DM interpretation, then it's more amicable to homebrewers. The important thing is that elves hate orcs, not the reason why. The important thing is that tieflings are fiendish humanoids, not any particular justification for that. 4e may very easily focus on the justification, rather than the important thing.