Imagine what it is like being a person who actually committed a crime but they are denying it. If then the police show the cadaver dog barking in the same areas that were involved with the crime, what emotional pressure that would have on the criminal's conscience? It wouldn't matter if the dog was wrong or it was an alert from another body it will still be effective only if the dog alerts in the right areas.
That might qualify as legitimate bluff. The dogs did alert and the PJ no doubt thought they'd finally catcheed the monkey (s) even though they didn't know themselves what was within the dog's "training parameters". We still don't, in fact, although the Met might.
Faced with "evidence", some crack and admit their crimes (which, if Kate's account is accurate, was to present the lawyer with the videos and to advise her to admit to whatever, presumably crimes with the prospect of a lower level of potential prison). Some falsely admit to crimes (I find the Innocence Project interesting) for all kinds of reasons; some simply change the story to fit in with whatever evidence they're presented with.... the list goes on of various shades of innocent to guilty.
I haven't found anything to confirm that either of the McCanns were able to read the DNA results - which appears to have been the other "evidence". At the time, it could only have been the Lowe e-mail, not the formal final report that was sent later along with various other bits and pieces from the HO.