The authors provide no evidence that this murder would or could have been solved but for encryption. It is a tacit appeal to emotion seeking to play on the revulsion of murderers that a non-technical audience can identify with while they downplay real risks that don't evoke the same strong emotions so that they will accept compromised security.
And if we could see inside people's minds crime could be solved as well. Your parent's comment is spot on, nothing has changed at all, just now the police can blame murders they have no intention of solving on Apple/Google.