This statement is based upon a terrible misunderstand of Postel's
robustness principle. I knew Jon Postel. He was quite unhappy with
how his robustness principle was abused to cover up non-compliant
behavior, and to criticize compliant software.
Jon's principle could perhaps be more accurately stated as "in general,
only a subset of a protocol is actually used in real life. So, you should
be conservative and only generate that subset. However, you should also
be liberal and accept everything that the protocol permits, even if it
appears that nobody will ever use it."
Further discussion on the topic: