I never saw MacKenzie Phillips with Dr. Drew when it originally aired, but I am now through reruns.
Knowing the info I know now and see her in rehab (and the causes) makes perfect sense. Though she didn't OFFICIALLY go public about her relationship with JP during her stay on the show, she is saying LOUDLY between the lines in her therapy sessions what the true story is/was.
Though I never had any doubt that she was lying before, it REALLY is concrete when watching her on the show.