As quoted in the New Testament, Luke 4, Jesus read from the book of Isaiah when He was in the synagogue at Nazareth, showing that they had a copy there too. That was a hundred years later than the Dead Sea scroll but you can be sure that if they had a copy of that book in Nazareth they had a copy of it in every synagogue in the whole scattered Hellenistic world, and there were a LOT of synagogues there, as you can find out by reading the book of Acts, as Paul took the gospel to them on his missionary journeys. You can be sure they didn't get their copies of the scroll from the Qumran community.
That's evidence that the scroll was well known and well distributed in Jesus' time, which suggests something a lot older than a couple hundred years, and fully recognized as the word of God, part of the standard Jewish canon.
And besides that, He is reading from a portion of that book that prophesies of the coming of the Messiah. He reads the section and then He tells them He Himself is the fulfillment of that prophecy:
Luke 4:21 And he began to say unto them, This day is this scripture fulfilled in your ears.
So even if you absurdly prefer to date the book far more recently than either the ancient Jews or the Christian Church dated it, you can't date it after Jesus' announces the fulfillment of its prophecy in His time.
And again, it's absurd in the extreme to think you can date any of the scriptures from the last extant copy known.
Edited by Faith, : No reason given.