I don't give this reminiscence ANY credit. The very first version of Postel's law makes it VERY clear that Postel intended his law to deal with non-compliant behavior in a tolerant or "liberal" way: "In general, an implementation should be conservative in its sending behavior, and liberal in its receiving behavior. That is, it should be careful to send well-formed datagrams, but should accept any datagram that it can interpret (e.g., not object to technical errors where the meaning is still clear)."
Nice to see that so well documented: I'd only seen the one most common reference, RFC 793.
That said, it's still unclear how far this extends: the example given is of an unknown error code, which might lead you to think that the requirement is "syntactically well-formed input where you can't 100% determine the semantics." That's a far cry from the way browsers handle malformed HTML. Similarly, you have to apply some judgment concerning what an agent can interpret the meaning of.
See my potted history of Postel's law: http://ironick.typepad.com/ironick/2005/05/my_history_of_t.h...