As a JW, I would've said the same thing. Historians are not doing guesswork. And the Bible itself points to 587 BC.
That is why I did this video series on the subject exploring this from every angle.
https://www.youtube.com/playlist?list=PLyNx0oM_bmgDJHQFzgm4L0rRuPpwTOVmz
https://www.youtube.com/watch?v=r-K8pyVfKcU&index=1&list=PLyNx0oM_bmgDJHQFzgm4L0rRuPpwTOVmz