How is that relevant? Whether or not employees unionize and how they do so is up to them to decide. It still should be illegal for Amazon to interfere in that process in any way.
If employees don't agree on stuff to the point there's a 50-50 divide, then they aren't actually united. There is no union. That's fine, as long as Amazon accepts whatever result. It's also totally possible that 99% want to unionize so they can apply maximum leverage on Amazon and extract better pay and benefits out of them.
A fair chunk of southern and western US consists of "right-to-work" states, in which one cannot be compelled to join a union as a condition of employment.
