In other words, "commoditizing the complements."
As GitHub makes certain parts of the software industry significantly more accessible or cheaper, its complements will succeed commensurately. For example, easy access to, and encouraged proliferation of, skilled talent and robust open source (free) software.
That GitHub provides infrastructure for other startups makes sense, but I'd strongly push back on the idea that VCs will invest in a particular company like GitHub primarily because they think it will improve their investments elsewhere. That's a leap in market forecasting, and the simpler (Occam's Razor) and more rational explanation is that they expect(ed) a good return with some added benefit to "the ecosystem."
I'd be more in favor of this argument if you restated it slightly as, "Investors like investing in companies like GitHub because they can initiate feedback loops with their existing investments that result in mutual prosperity."