That's the other thing: this new "revelation" is about something discussed during production of the movie. The Lion King is no different than any other movie in this simple regard: before the final product is actually finished, ideas are created, discussed, modified, and finally, left in or thrown out entirely.
As it was already pointed out, Scar wasn't originally intended to be a member of the pride, but an outside rogue. That idea was quickly thrown out to make the story more personal by having the main villain be within the pride itself, and not outside of it. That being said, it's easy to see how this original idea could have lingered in the production team's minds, thus giving us this "Mufasa and Scar are not brothers" 23 years too late.