Here's a summary of their proposal:
We suggest that following through the logic of this chronological approach to BH actually leads inevitably to the conclusion that all the biblical texts were composed in the postexilic period, which is exactly the opposite of what its proponents have claimed. Now, this may in fact be a conclusion which is congenial to some. But others will not find this agreeable, so we will offer a way out of this conclusion by arguing that the presuppositions of the chronological approach are undermined by the evidence. On the contrary we will argue that the best model for comprehending the evidence is that ‘Early’ BH and ‘Late’ BH, so-called, represent co-existing styles of Hebrew throughout the biblical period. Then we will deal with the objection that Persian loanwords are an irrefutable proof that the chronological approach is correct. Finally we will step back and ask some hard questions about the presuppositions involved in the dating—by linguistic or other means—of the books of the Hebrew Bible.