Josephus wrote before Yavneh.
The Sanhedrin moved to Yavenh (aka Jamnia) after the destruction of the temple in 70 AD. Jospehus wrote his Antiquities of the Jews at the end of the first century, just about the time when the phantom "council" took place.
Late first century developments attributed to Jamnia
Today, there is no scholarly consensus as to when the Jewish canon was set. Nevertheless, the outcomes attributed to the Council of Jamnia did occur whether gradually or in a definitive, authoritative council.
I'm really not trying to be argumentative, just collecting information in bits and pieces, to be added to in the future whenever it comes up; lots of what I know I consider known only provisionally -- I can live with a certain amount of uncertainty (lucky thing!)!