You're missing the point. In her mind, there is in fact a probation order requiring her to submit to testing. We don't know if she was on Flexeril or not that night. She said she was. So what? She also said she was raped. Don't you think that someone who lies about being raped would also lie about what drugs they were on, and claim that whatever she was on was legal, especially is they went so far as to lie about a non-existent rape in order to avoid being tested?
No doubt she had a prescription for the muscle relaxant.