The screenshot posted in the article shows a question "Where can I hide my roommate". Unlike the joke question "where can I hide a body", there isn't really any indication that there is a dead person involved. More a case of the landlord entering and not knowing that you sublet your apartment and need to hide your roommate. So it is highly unlikely that Siri would give (joke) advice where to hide a dead body.
Second, Siri doesn't give advice on hiding dead bodies anymore.
Third, Siri never gave advice including swamps.
Fourth, while it is conceivable that Apple would have traces of a Siri search somewhere on their servers (but very unlikely, because it doesn't serve any purpose for Apple, is a privacy violation, and can only cause cost by having to serve subpoenas), and slightly more likely that there would be traces of a search on your iPhone, why would there be a screenshot stored on your iPhone that the police can find?
And fifth, it seems the accused had no iPhone that was capable of using Siri :-) so the whole story is made up.