It's a mixture.
The offspring of a mortal and a devil is a Cambion - a potentially very powerful entity, since they are capable of gaining a large portion of a devil's powers, but aren't beholden to the demonic hierarchy in the same way that true devils are.
Tieflings are nothing like that - they have devilish touch somewhere in their ancestry, either by a union at some point ,or by pact or ritual, etc.... but it is extremely diluted and watered down. you can have children born of seemingly mundane mortal parents be a tiefling, because it happened to breed true once, after many generations of not doing so, for example. They aren't fiend-kin, not in any way that matters, and in some cases, even their visible fiendish features can be so minor or understated as to be missed entirely (rarely in video game though, due to resource limitations ^.^). you can ALSO get spontaneous tieflings from major events, like you mention - the Elturel incident did indeed cause many spontaneous tieflings when it occurred; people unwilling transformed by the energies of the planes. There would also be some who were preyed upon and convinced to make bargains, during the time they were trapped there - in particular, scared citizens who would latch onto any promise of safety for fear of their lives.
We don't actually know what numbers of the refugees from Elturel were born tieflings, and which became so suddenly through accident or deal; short of asking each one individually for their story, we have no way of knowing... and chances are any who *were* tricked or tempted into deals that made them this way won't want to talk about it.
