Let's remember that Larry wasn't the first to say this, which is part of why he says it's understood instinctively.
John Postel writes in Internet RFC 793, defining the TCP
2.10. Robustness Principle
TCP implementations will follow a general principle
of robustness: be conservative in what you do, be
liberal in what you accept from others.
This is a document from September 1981, and lays one of the cornerstones for the modern Internet. If this was not laid forth in that document, TCP/IP may not have become any more standard than DECNet or AppleTalk.