It’s perfectly reasonable for them to use 586 BCE as the anchor date, because from their perspective it is a verifiable and reliable date to start from. I guess the destruction of a city is far easier to identify by archaeological means than the return of Jews a generation later.
What you can’t do is use the 586 BCE date as the base date for the method and then turn around and say the 586 BCE is supported by the method, because that’s like trying to hold up the branch your sitting on. (If I’ve understood what the article says) I don’t think they are claiming the method does support 586 BCE, because that’s a peculiar JW interest anyway, they would have no interest in doing so.