According to Wikipedia, Musk is not only chairman of Solar City but also "remains the largest shareholder" - so the line from the article while lacking detail, is mostly correct.

It's a good risk distribution strategy to split your various investments into separate corporations and then do business deals between the component organizations. But it can also be a profitable tax strategy when there are differing tax implications or accounting rules depending on how deals are structured. The rules surrounding the tax and reporting implications of buying versus leasing are actually one of the current major topics in the US accounting governance world. (My wife teaches Theory of Accounting which goes deep into the frustratingly fascinating details of how to appropriately and clearly represent expenses and earnings for management, investor, and government audiences. It's fascinating stuff, but confusing for an outsider.)