The distinction here is that this lie is so coercive that it's unethical to allow any confession or evidence following it.
What would you do if you don't have money for a lawyer, and the police say that your unassailable DNA evidence was found on a now-dead acquaintance of yours, and your options are confess to the murder and get 10 years with opportunity for parole, or fight and get life?
Even if you knew that the police can lie, know you are innocent but can't prove it, and you believe that the DNA test is real or the police are willing to fake it, you might confess b/c that's the best option.
What would you do if you don't have money for a lawyer, and the police say that your unassailable DNA evidence was found on a now-dead acquaintance of yours, and your options are confess to the murder and get 10 years with opportunity for parole, or fight and get life?
Even if you knew that the police can lie, know you are innocent but can't prove it, and you believe that the DNA test is real or the police are willing to fake it, you might confess b/c that's the best option.