From JJ Cohen:

The giants are an ancient, vanished race whose fossilized remains are not mysterious bones or odd topography, but the lingering worship of their iniquity. The references to constructing idols and deifying the sun and moon which follow make it clear that Ælfric has both biblical and classical deities in mind. By describing the genesis of the false, mortal divinities of the Greeks and Romans (along with those of the Babylonians, Canaanites and wayward Israelites), Ælfric is repeating a connection frequently made in Old English literature between the opprobrious giants of Christian tradition and the gods of classical mythology.