How do you define a co-founder? Tesla was just an empty shell before Musk and JB arrived, so I think they are co-founders of Tesla, but not the only co-founders.
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).