Did he really? I have this from Justin Martyr:
"I and others who are right-minded Christians on all points are assured that there will be a resurrection of the dead, and a thousand years in Jerusalem, which will then be built ... For Isaiah spoke in that manner concerning this period of a thousand years."[Justin Martyr from A Dictionary of Christian Beliefs. David W. Bercot, Editor]
And this also from Justin Martyr:
""There was a certain man with us, whose name was John, one of the apostles of Christ, who prophesied by a revelation that was made to him, that those who believed in our Christ would dwell a thousand years in Jerusalem".
By most accounts Justin was what we would call premillenarian.
As my quote indicated Justin believed there we other sincere, orthodox Christians who held another view of the millennium
Also, do not confuse Justin's premillennialism with modern dispensationalism.
Justin believed that the Church was the fulfillment of all the promises to Israel, basically the subject of his Dialog with Trypho.