From the article:

Suster points out that the longer the company stays private, the more valuable it becomes. And if during this time VC’s can hold onto their pro-rata (fancy word for what percentage of the startup they own), they can make a ton more money.

The premise of Growth capital is that if that by staying private longer, all the growth upside that went to the public markets (Wall Street) could instead be made by the private investors (the VC’s and Growth Investors.)