Did the police not have records of all the calls Amanda made the day of the murder, right down to the exact number of minutes that have been the subject of many debates? If they had access to those records, how hard would it have been to get access to her phone records for the day before, if they wanted to? Don't cell phone records usually list the numbers that have been called? It is information the phone provider can track and provide to the police, at least in the United States.
If they didn't have access to the text message, they wouldn't have asked her about it. If they had access, they knew who it was sent to.