The confession is admissible as evidence, it doesn't result in an automatic conviction. Now that the defence knows about the stingray they will paint the rest

The confession is admissible as evidence, it doesn't result in an automatic conviction. Now that the defence knows about the stingray they will paint the rest of the case as fruit from the poison tree. They just need a reason to call the officer and ask about the stingray. The prosecutor can't withdraw the defences' questions. The defence will merely reintroduce the evidence as evidence of police misconduct, this isn't going away anytime soon.