I didn't say it was 'self-evident'. It relies on the proposition that market participants in the bond market do not have any incentive to alter their purchasing behavior in the hope that the Fed can actually accomplish its goal of meeting an inflation target of its own setting. I would say that given Greenspan's contact with these individuals on a frequent basis, he probably understands that he cannot fight the bond market. If he understands this, yet does not admit that even his most unconventional measures have little to no hope of succeeding, I call that dishonest.
No need to appeal to 'self-evident' axioms or truths at all, just common sense.