There is no reason to name her as a POI, as what difference does it make? They clearly do not have enough to arrest her. I think they are still indicating that she is, without having to take an official stance on it, in case they are wrong.
Also, what I read from what he says/does not say, that Terri is the likely the culprit, since he can't/won't go there, it leaves room for those wildly improbable theories floating around that Kyron is okay and someplace else, being help "captive"-I don't believe that at all, but since Gates can't just say no to those questions (because he can't prove it yet) he can't shoot down those theories in public either. (goes to the defense, if LE locked into one thing too early and only focused on Terri.) I think this is also why he is saying they are assuming Kyron is alive, so that they do not have to name this a murder investigation yet. All semantics, as usual, with same result, for me-they are looking at TH.