I guess I don't see how this adds to Demandred's character development at all. This says almost nothing about Demandred and a lot about Rand. We already knew that Lews Therin was arrogant. It doesn't give Demandred any less of a cartoon bad guy reason for becoming what he has become.

Demandred is still the comic book character that went over to the Shadow because his pride couldn't stand being in Lews Therin's shadow any longer. So what if Lews Therin was kind of a prick in the way he kept outshining him? Demandred's motivation for trying to bring about the end of all creation as we know it is still that Lews Therin was a little bit better than him and a prick.

Demandred may turn out to be a highlight in the book, but only because he has perhaps laid some awesomely competent plans, not because his backstory is suddenly richer.