Earlier quoted context omitted.
You are right, but your parent is right too. If the police logged into his Kik account from his device, they would have seen the lack of connection to the case. If they simply trusted the Kik account contents from the subpoena response, then it was up to the defendant/experts to identify the issue and rebut the evidence.
IDK if you know much about kik but the only way this makes any sense is - I suspect he might have had other incriminating messages on his actual account he chose not to reveal.
But also, I read the article where it said that no corroborating evidence of any crime was found when searching his house or devices, so I suspect his Kik account is not likely to be full of illegal content if his other possessions weren't.