God himself is the temple of the holy city, the New Jerusalem (Revelation 21:22). His presence, then, fills the entire city. Yet God—and hence his throne—is not to be identified with the city as if throne and city are the same; instead, the throne is “in” the city. The point though is that the Lord God (and the Lamb) are so fully sovereign that his Kingship dominates the city fully, as the next phrase makes clear.
3 No longer will there be anything accursed, but the throne of God and of the Lamb will be in it, and his servants will worship him.