IMO it is this way: When this initially happened, the detectives were still in a state of "we are not quite sure yet about what really happened here" and they were being accosted by the media, left, right and centre. They have to be very careful not to reveal what they know. I believe the "no foul play" statement may be a bit of a "don't want to answer specific question from you, journo, because we have a pretty good idea who killed her". The "no foul play" IMO is also an underlying message to the perp meaning "this was no stranger that killed her".
If I remember correctly, very early in the investigation, GBC was mentioned as a "suspect/POI", but it was quickly corrected to be "no comment". Not everything that has been said on camera (or the press) is totally correct, i.e. some are still reporting she went for a walk at 10pm.