A client-services model isn't a good fit for a startup, due to the iterative nature of a product in search of a business model. The entire article implies a waterfall approach where a designer goes into a cave, does some work and presents it for "review".
Instead, a good "startup designer" will be an empathetic generalist (UI/IA/Front-end) with good market sense. It's important that they're comfortable skipping a lot of "design process" and rapidly iterating with engineering (their "familiarity with flat design" notwithstanding).
Also, many great designers don't have portfolios. That's because they're busy doing real work. Ask to see real work.
Despite the linked article, if you're early stage you should go unicorn hunting. It's critical to hire from the A-list for the inaugural team.