To sign on the dotted line means to indicate one's agreement to something, but in this context the person is disagreeing with the half-truths.
Context: "He knows how to dangle half-truths in front of cops and das and get them to disagree with him."
Hypothesis: He knows how to dangle half-truths in front of cops and das and get them to sign on the dotted line.
Based on the context, is the hypothesis true, false, or unknown?