Isaiah's reference to foreign peoples is here stressed by specifying that they are "fere" = fierce, wild, savage, and "diverse" = different from us, alien to us, strange. The wording echoes Dante's powerful description of Cerberus in Inferno 6: "fiera crudele e diversa," literally: a cruel and strange, or rather monstrous, wild beast.
And so, by twisting Isaiah's text and adapting it to 16th century audiences, the poet makes us remember the first impression that Native Americans left on European explorers, then conquerors, then readers. But, in the case of Tasso, it would be wrong to see this as plain racism; rather, it is about theological imagery identifying idolatry and beast-like (mental) attitudes, as St Paul did in Romans 1: 18 ff on the basis of several passages in the Tanakh (Jewish Bible), e.g. Psalm 135.
Tasso's varied views on Native Americans / Indians will be more thoroughly studied on some future occasion.