That's not the argument. He's not upset Arduino was a huge success, nor is he all that upset that he is not involved with Arduino (as far as I can tell from this article, as it's written very professionally). The argument is that there is a rather significant fight between the co-founders of the Arduino project about who owns what of the Arduino brand. This article paints a picture that in reality, the product behind the brand was essentially ripped off of someone else's hard work with very little contributed back to the original project. I don't think anyone wouldn't start to feel rather frustrated if someone forked their open source project, made it hugely successful (while complying with licenses and other conditions, but without directly contributing back) and then started fighting within themselves over who "started the project all on their own".