Unequal enforcement of company policies is not working conditions?
Forcing a trucker to drive 20 hours straight is working conditions. Refusing to install proper lighting in a warehouse is working conditions.
Having a policy saying "don't be an asshole", and then enforcing it in a way that is perceived as unfair, is not working conditions.
If the letter had directly alleged that Musk or other leadership was abusive towards the employees, they would have a case. But just saying "we thing Musk is an asshole, and we have a no-asshole policy" is not protected speech.
Many employment laws just create causes of action for civil litigation. I.e. they define types of harm for which the employee can seek compensation in the courts.
Very specifically for this case, they protect the right of employees to talk to an employer about improving workplace conditions. With or without a union, and with or without any interest in unionization.