The point is that all of the possibilities are on the table, as of 5e, Sai - if you want your teifling to be tiefling by direct blood lineage that's perfectly acceptable, and is just one of the possibilities; there are other possibilities, and other ways it comes about, but they didn't say no to the original ones. If you prefer your tieflings to have evil in their blood, intrinsically, you can have that too - though if you're running in a world that is based on the current realms lore, you have to understand at a player level (not necessarily at a character one) that that won't be the case for all tieflings you may meet.
