You are mistaken. For example:

- He wrote Slate's economics column, "MoneyBox," for a number of years.

- He wrote a book about the economics of the housing market.

- He co-founded Vox.com, which -- presumably -- reinvests its profits to grow the business.

I don't personally know much about him, but your first two points do nothing to make him look less like a political pundit.

I submit the points address questions over his economic analysis credibility.

I'll add that his writing was good enough to trigger a response from Bezos in a letter to shareholders.

The book, as someone who is generally a fan of Yglesias, is around 35 pages long and is very light on actual economics and heavy on rhetoric

Yglesias has more than a passing grasp of economics, and I guarantee that he understands reinvesting profits to grow a business. It's not that hard a concept to grasp. GP's quote was deliberate hyperbole, IIRC, and was not meant completely literally.