Sinek argues that most companies lose their purpose and deviate from their core business (and potentially into unprofitability/inability to launch new products and services successfully) because they focus too much on 'what' they are selling or 'how' they deliver to their customers. The solution to this is to start with 'why' - having a core purpose that will drive all other aspects of your product/service and delivery.
The author of this article asserts that this leaves out a 'who' - the customers that you are selling to. He says that one needs to begin by choosing your target market and aiming all efforts tailoring the experience to them.
I think this is a bit backwards - especially when he is trying to suggest that Sinek is "fundamentally wrong". To me, it would seem that if you start off with a higher level purpose, then the 'who' will fall out of that. I want to manufacture and sell a completely open source mobile phone because I believe that all humans should have access to commodity communications devices that aren't tied to a proprietary OEM or service carrier. The 'who' from this will come out as either 'people who have an interest in open source and freedom' or 'people who want a cheap commodity device'. How do I pick between these groups to focus? I go back to my 'why' and elaborate it. I don't pick one arbitrarily (i.e. start with 'who') and then pick a raison d'etre based on that choice.
I think it's also fair to point out that Sinek seems to have a weird fascination with trying to back up his points with Bullshit science (the TED talk mentioned in the article tries to assert that his methods are correct 'because of biology' which is tenuous at best. He makes many similar claims), but overall I think the 'golden circle' of starting with a purpose is fundamentally sound and absolutely not disproved or even really contested by this article.