I think it's more that if your police were known for beating confessions out of suspects, when they presented your signed confession to the judge, he wouldn't necessarily think that you signed it because you committed the crime.
But yes, police officers often ignore confessions when they don't have good reason to think that the person committed a crime. Often, when a crime is reported in the media, many people will come forward and confess... not because they actually committed the crime, but for psychological reasons: either because they falsely believe that they committed the crime, or because they feel guilty about some non-criminal act and believe that they need to be "punished" to be absolved of their guilt.
And in this case, even if a confession of "working with the devil" was made freely, it wouldn't be proof of anything beyond an overactive imagination.