While not one of the original co-founders, Musk has devoted enormous amounts of time, money and energy into Tesla since nearly the beginning, which I think makes him close enough to a co-founder. If you read Arrington’s post, he even says the same about Heather —- not technically a co-founder but one in practice given the amount of work she put in.
The guy he is calling out is alleged to have put zero time or money in — he just grabbed what he could.