A score is twenty years. So Isaiah is prophesying of the downfall of the northern kingdom. This occurs at the hands of the Assyrians during the reign of Pekah's successor, Hoshea. This prophecy is fulfilled well within the 65 year time limit.
'Then the king of Assyria came up throughout all the land, and went up to Samaria, and besieged it three years.
In the ninth year of Hoshea the king of Assyria took Samaria, and carried Israel away into Assyria, and placed them in Halah and in Habor by the river of Gozan, and in the cities of the Medes' (2 Kings 17:5-6).
"In this prophecy, Isaiah promises that the Syro-Israelite alliance will fail and that Israel will be scattered within sixty-five years. The fulfillment came about in successive stages. First, Tiglath-Pileser III (Pul) attacked Syria and Israel in 732 B.C. and took many Israelites captive to Assyria, especially those from the northern tribes. Secondly, in 730-727, Pul annexed the Transjordan area and deported large numbers of the Israelite tribes from that area to the far reaches of the Assyrian Empire. Third, in 726, Hoshea refused to pay Assyrian tribute, and Pul's successor, Shalmaneser, retaliated by attacking Israel and besieging Samaria, which fell in 722 B.C. Thus, within a dozen years of Isaiah's prophecy, the alliance had completely failed, and three major groups of Israelites had been deported. Finally, large groups of the Israelites fled from Assyria to the remote areas northward and became the lost Ten Tribes of Israel. Apparently, within about fifty years of their leaving Assyria, they were scattered so widely that many of them no longer existed as a cohesive group. Thereby Isaiah's prophecy to Ephraim was completely realized." (Victor L. Ludlow, Isaiah: Prophet, Seer, and Poet, 141 - 142 as taken from Commentaries on Isaiah in the Book of Mormon, ed. by K. Douglas Bassett, [American Fork, UT: Covenant Publishing Co., 2003], 129-130)