The more apt framing of this article would be something along the lines, of "The Agency Trap." The author grew his marketing agency to $1M quickly. That might sound great for HNers here, but there are reasons why such businesses don't attract handsome valuations. You always have to keep grinding to keep the inflow of customers, you're never free, and growth is directly tied to the number of employees. Additionally, such business can have severe ups and downs depending upon market conditions.
Building a product, on the other hand, is slow at the start but after a point, the rewards start showing up. The inflow of cash is consistent and keeps growing while the costs remain the same.
There's nothing wrong with the agency business, but it'd be a smart bet to use at least some of the proceeds into building something long-term. Some agencies, notably 37signals, have been quite successful doing that. Most, though, never do it.