California can make its own police officers wear uniforms and provide identification, but it can't make the federal government do the same. Even within California.
That's different than traffic laws. Traffic laws apply to everyone, so California can (maybe) require that federal officials comply with them. It's true that the California No Vigilantes Act applies to all officers too (including California officers), but since it "targets" federal officers, it's barred by the Supremacy Clause.
So holds the Ninth Circuit, which now reverses the lower court's refusal to grant a preliminary injunction. (A decision it has already stayed, so the handwriting was already clearly on the wall on this one.)