I think mistake is a strong word. It clearly wasn't part of the original plan, at least where it was, and did take (and continues to take) a lot of time and effort on the part of Tesla to get it to market already. However, Tesla was moving into SUVs eventually, they would have to, and the X if nothing else provides a ton of useful data and experience to them down the line.

Best idea? Probably not and maybe a little premature too, but the article makes it sound like Tesla's going out of business because of it and I don't think it's nearly that bad.