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 protocol:

    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.