JEB
Legend
There's precedent for this in 5E: Ghosts of Saltmarsh has tieflings, but they're tied to Iuz. They could certainly provide some explanation for dragonborn (@Remathilis listed a bunch of good options).Maybe. I would want the setting material to tell me which it is.
Interestingly 4e, for all its controversies about adaptations of older settings, also usually tried to find some rationale for its introductions of 4e core options.