Credit is like karma. It's good qualities that people have, often related to the way others perceive us. We "give credit where credit is due", meaning that we recognize someone's hard work and achievement. The bank may give us a credit rating, which says whether we have enough good qualities (job skills, responsibility, etc.) for them to believe that we will repay a loan. "To his credit" means that a person has done something which adds to our good perception of them; he has done something that we can respect.
In this context, the writer says that he thinks it was a good idea for the director to refuse to defend or justify the film. If he had, we could not respect him. Because he did not, his good qualities have increased in some measure: we can respect him at least for this one action. I can't think of another phrase that would mean quite the same thing, though you could try:
Thankfully, he refused.
I respect the director more for refusing to do this.
I think the director showed good judgment in refusing.
- For Teachers