walls of jericho said "Divorce is not unscriptural".
That is incorrect. Besides my hating that word 'scriptural', it does say that Jehovah is hating a divorce so he does not want that.
It is just that legal action does not break the marriage ties. But when one has sex with another that breaks the ties because it forms new ones.
That is what happened in Israel. They didn't have weddings. They had sex and that made them married.
So by Jesus' words, the divorce would make the one feel free to remarry but that would not not be free. Then that 2nd marriage would break the first bonds and form new ones. It would not be an adulterous marriage as there is no such thing. Only that moment was adulterous.