I think Zumwalt got to know Kerry after he became a Senator. Zumwalt had his own political ambitions so he was just spouting the party line in support of Kerry and his naval service. As a former naval officer who served in Vietnam 1967-68, I can understand Zumwalt wanting to support a comrade in arms, especially one who was now a senator. I doubt Zumwalt was that acquainted with Kerry's antiwar activities and associations.
At least now we know why Kerry believes he is insulated from any attack on his war record. But you can bet he is not releasing his service record because there is stuff he doesn't want made public.