Another thing is if nature is used as inspiration for the story. In reality even if one lion is more dominant than the other, they share the responsibility of being King. That means they they share the responsibility of guarding and defending their territory. As well as sharing the uhh...
privileges extend to them by the lionesses. Therefore Scar would have had less reason to bump off Mufasa because getting rid of the powerhouse on your team means a lot more responsibility would then fall on you. It would be a bad move on Scar's part as he would have more to loose than he would to gain.
Also in one of the original concepts of the movie it is mentioned that Scar was originally supposed to be a Rogue
http://lionking.wikia.com/wiki/Scar (check the In Development Section)
Also in one of the Making Of The Lion King Specials (I don't know what one)
https://www.youtube.com/results?search_query=the+making+of+the+lion+kingIt is mentioned by either/and/or Rob Minkoff, Roger Allers, Don Hahn. That they decided to have Scar be Mufasa's Brother/Simba's Uncle because that made the threat more menacing if it came from within the pride. For me it also explains why Mufasa did not realize just how dangerous Scar was or why none of The Pride. Especially Sarabi didn't suspect Scar of treachery when Mufasa (and supposedly Simba) was killed.
For them to say, suggest, or imply that Mufasa and Scar were not related by blood undermines the emotional impact that they were going for and that of the story itself.
There is also The Lion Guard which Don Hahn himself (for lack of a better word) gave his blessing when it was previewed at D23 in 2015. Taking that series as canon (as I believe they are trying so hard to establish as) The Roar of the Elders is inherited by the 2nd born cub of The King And Queen of Pride Rock. Therefor Mufasa and Scar have to be brothers by blood.
As for the article itself, it sounds very fishy for all of the reasons I have mentioned.
This is just speculation on my part, but I'd be inclined to think that during the interview ideas that were tossed about in the creative process were shared. Somewhere along the line the reporter misunderstood what ideas were rejected and thought that the film creators had different intentions than what was apparent in the film.
More speculation on my part, but I think the reason Disney didn't do anything about this article is because that it is such a small piece. All Disney wanted from it was to plug the fact that The Lion King is coming to Blu-Ray on August 29th. They
may not have cared too much about the articles content as long as there were no glaring falsehoods.