If you've got partners and plan to grow and/or are planning on hiring several employees in the next few months, you should do the couple of hours of homework and lawyering it takes to file the right papers at the beginning.
I should have been more specific and said that the fact that they neglect to mention what you should do when it's time to grow, and only recommend creating a legal entity past the 5-year mark, tells me that they're definitely not focused on HN types as their target audience. They're targeting the self-employed.
(Which admittedly is a bit harsh on the HN crowd)
My perspective (which I was mistakenly projecting on the rest of the community) doesn't have much room for being self-employed doing projects for other people as a long-term goal. But that's the angle the article takes I think.
I can see that some people here might see themselves staying a sole proprietorship for 5 years, and creating a lot of value in doing so. Just not my cup of tea.