I think the point is that the message history would show incriminating messages.
He'd have been saying "this isn't me", but somehow no one noticed it was the wrong username.
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.
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.
Because the message history was for his actual account, not the criminal's account.
The whole point is that they looked up the wrong username. And when they looked it up, they found an innocent man's identity, but didn't bother to look at the message history, which would not have had any of the messages.
Which is super-weird, because if he'd been messaging this one child, you'd think they'd be looking at all of his messages to find potential messages with other children. And then immediately noticing the lack of the messages that started the investigation in the first place.
Still pretty flimsy evidence when they had voice recordings and pictures sent by the real culprit that surely didn't match the accused.