We don't know if what Thomas said is truthful, but I would bet as the facts unfold and LE makes them public or during the trial, the truth is closely aligned with what Thomas has told police. Why be silent for 6 months, then suddenly turn yourself in with all this info? She didn't put the blame solely on someone else, she is taking the fall for her part IMO. Of course, I would bet LE already knew this but didnt have enough proof (couldn't prove who was sending the texts, didn't have a body, etc).
What's puzzling to me is why they did what they did, if in fact her story is true.