What does Siddhartha mean in the last chapter ("Govinda") when he says that "the opposite of every truth is equally true!"?

The statement about truth as being one where its force and
counter-force are equally present in consciousness helps to bring about the realization
of the transitory nature of life.  The "opposite of every truth" being "equally true"
helps to reinforce that life is a journey, something that is ongoing.  It is one that
Siddhartha has fully understood as one where "truth" cannot be grabbed or held on to
without understanding its own sense of being larger than one's own sense of existence. 
For Siddhartha, there is a full understanding that the nature of existence is one where
truth is evident in everything.  There is not a narrow or pedantic view of being where
one element is "accepted" and another "rejected."  Rather, Siddhartha has begun to
understand a unity that is present in consciousness, one that argues that there is only
truth present.  In what is accepted and what is rejected, there is truth to be grasped
and understood.  In this, one no longer concerns themselves with "being" right and
rather understands their own duty as being one to embrace right.  In this, truth is
multifold and complex, something whose full implications might be outside that of human
grasp.