A scab is hired to replace someone who is striking. He had previously (as in years) been hired to replace Bialik when she was unavailable. Due to her commitments to the strike, she is unavailable. He, not being a member of the WGA or SAG, has no commitments to the strike, but does have commitments to his contract, which states he is to replace Bialik when she is unavailable. Does that make sense?