Regarding Lord Soth's history, the reason the article is so off is that it's apparently drawing from Edo van Belkom's novel Lord Soth, which is, in my opinion, a fairly poor novel.
The book takes some liberties with Soth's history, getting things wrong ranging from his hair color (he's blond, not black-haired as the book says) to how Paladine tells him to stop the Kingpriest (it's supposed to be that Soth has a vision from Paladine, but that book says that Paladine spoke through his wife, and Soth later wondered if she was faking it).
The bit about his child is that, in van Belkom's novel, Soth's first wife took a magic potion to help her conceive. The witch who gave it to her warned her that the child's conception would reflect Soth's virtue. Since he was already cheating on her at the time, it was born monstrous, and as his wife accuses him of having done something, he murders her and the baby.
None of the above matches with anything else regarding his history, as far as I know. Ironically, van Belkom referenced James Lowder's Ravenloft novel Knight of the Black Rose quite a bit for backstory (such as Soth's senechal, Caradoc), but still managed to ignore a lot of what that book had already laid down regarding Soth's past.
In any event, that's where the confusion comes from.