And they all leveraged some incredible pieces of infrastructure that were built up for them over several years by those with more experience, the primary examples being the Linux Socket library and the TCP/IP protocol.

Building an application that can "scale" doesn't mean it's well built. Modern software engineering sits on the shoulders of giants, especially older ones who came out of research labs.

As other people stated: they build standing from the shoulder of the giants.

And even if they had started from zero, these guys are the exception, not the norm.

We can find 20 year-old geniuses in every camp.

I'd be interested in a field where someone wasn't standing on the shoulders of giants.

Yes, yet I wonder how good they would stand if someone did a proper Software Engineer review of their architecture and code quality.