I know some people don't think lie detector results really mean anything, however one aspect of taking these tests is the person being tested is asked pretty straightforward questions. We know both CB and DW state that they took these tests.
So, i feel that while the police interviews may have been more cordial where they were perhaps asked questions a bit more delicately (i do not know this to be the case, i am just saying, maybe). From my understanding, lie detector tests use yes or no questions frequently.
So, they may have outright been asked questions that are asking if they knew where SW was, did they put her these, is she alive, is she dead, did you do (thing causing death) to her. Again, these are questions I think maybe they were asked, these are not opinions i am putting forward.
Here is the opinion I am putting forward. They were able to withstand this questioning. At this point, LE seems to not be talking to them. LE can't learn anything new from them imo.