Yes Suzi, I believe it is binding as OSHA oversees all workplace safety. This IS historic. If they don't comply the employee is entitled to file a complaint with OSHA and that could bring the 800 pound gorilla into the room.
The only potential downside I can see is that employers may be more hesitant to hire a trans worker if they think it may cause bathroom issues. Let's be honest, they don't deny the job based on being trans but just say someone else was better qualified.
Pass comprehensive ENDA!