Everyone knew what case it was about- due to the twitter from President referencing Stone. Due to it hitting every news station.
When it involves a dramatic news story about 4 prosecutors leaving, resigning a case and misconceptions about AG handling a case, he is well within his rights to call a press conference to clear the air.
I'm not sure about that. The Twitter message posted around the same time about Comey, Strzok and Page seems to be absolutely more relevant to concerns that the AG might raise about doing his job right.