I read it as "...I have to think YC [can someday be] hugely important to that growth," not that it already was.
That said, although VC funding might be a small percentage of investment overall, it has a leveraged impact on the economy as a whole [0]:
"According to a study conducted at the Stanford Graduate School of Business, 43% of U.S. public companies founded since 1979 were funded by venture capital. These companies now comprise 38% of all employees, 57% of the total U.S. market capitalization, and account for 82% of all R&D spend — a proxy for further innovation growth — in the United States."
That said, although VC funding might be a small percentage of investment overall, it has a leveraged impact on the economy as a whole [0]:
"According to a study conducted at the Stanford Graduate School of Business, 43% of U.S. public companies founded since 1979 were funded by venture capital. These companies now comprise 38% of all employees, 57% of the total U.S. market capitalization, and account for 82% of all R&D spend — a proxy for further innovation growth — in the United States."
[0] http://a16z.com/2016/09/11/vc-economics/