When was the great synod, and what do you say to this part of the article:
During the first century, the Jews disagreed as to what constituted the canon of Scripture. In fact, there were a large number of different canons in use, including the growing canon used by Christians. In order to combat the spreading Christian cult, rabbis met at the city of Jamnia or Javneh in A.D. 90 to determine which books were truly the Word of God. They pronounced many books, including the Gospels, to be unfit as scriptures. This canon also excluded seven books (Baruch, Sirach, 1 and 2 Maccabees, Tobit, Judith, and the Wisdom of Solomon, plus portions of Esther and Daniel) that Christians considered part of the Old Testament.
Why did they meet if it had already been sealed?
The Great Synod was assembled at the end of the Babylonian Exile, at the beginning of the Second Temple period, about 450 B.C.
Why did they meet [at Yavneh] if it had already been sealed?
The meeting at Yavneh was a regrouping of the remnant at the time of the destruction of the Temple. They were under a severe calamity and the purpose of the meeting was to salvage what they could of the religious observance now that the Temple was destroyed. The rabbis at Yavneh anyway had no authority to make any changes since the chief rabbis of the time resided in Babylon.
I don't know what book you're quoting from, all I know is that this is not the Orthodox rabbinic scriptural history.