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.
I'd agree with you and say they're all cofounders (as, Wikipedia tells me, they settled out of court to agree on). But I think the media really emphasizes Musk, and in this article Straubel, as the co-founders while eliding Eberhard's contribution (ie. actually STARTING the company with Tarpenning).