It's good advice for 3 people with an idea and an application to YC. It definitely makes it easier for YC to set you up; we're in the process of converting from an LLC, and it is a huge pain in the ass.
It is one of the poorer pieces of advice on this site for someone who actually wants to run a company, and I wish YC would clarify it. To actually run a business, you need a incorporate for tax reasons, so you can invoice and be invoiced, so you can make binding, valid equity grants, and so your vendors, partners, and customers can't sue you for your house.
It will take you 2-3 hours, spread over 2 weeks, to set yourself up with an LLC and an EIN. It's not one of the Great Distractions you face starting up.