Elihu spent some time when he first began speaking to Job to establish himself as someone Job can relate to. By claiming to be perfect in knowledge, it sounds like Elihu is trying to elevate himself over Job like the friends did when they dismissed Job's speeches. Any counseling and consoling must be from a position of care, so this is confusing. But what Elihu is really trying to say is that his knowledge is like Job's knowledge, without falsehood but not infinite. Job already knows the things of God, and now he must trust that God has already made Job perfect through faith and will deliver Job from hardship.