I have also been a true crime obsessive since I could read a newspaper, yet I feel the precise wording makes a difference. An interview is a specific conversation that is recorded. If the Chief's 'an interview' does mean AFTER the arrest and not including any that occurred BEFORE, I feel that makes some sense. They could possibly have been talking daily with JS, but as official interviews go, maybe it was just the one.
She is apparently cooperating. If they wanted more interviews and she refused, would they say she was cooperating?
And, of course, there is the chance that she has had a complete mental break because I know if it was me, I would be in the biggest mental breakdown of my entire life.