I don't really consider that a particularly good source. Virgil says that Cacus was Half Human, in the Aeneid, Book VIII:
There was a cave here, receding to vast depths,
untouched by the sun’s rays, inhabited by the fell shape
of Cacus, the half-human, and the ground was always warm
with fresh blood, and the heads of men, insolently
nailed to the doors, hung there pallid with sad decay.
Vulcan was father to this monster: and, as he moved
his massive bulk, he belched out his dark fires.
Now at last time brought what we wished, the presence
and assistance of a god.
Additionally, I have been unable to find a single corroborating citation to your source about Medusa being Cacus's mother. Especially since, as a Gorgon, Medusa was a Daughter of Phorcys and his sister Ceto, ancient Cthonic deities, and was therefore not human.
Finally, your source says that Cacus inherited Monstrousness from his mother, but we know that other children of Hephaestus were also monsters, for example in Apollonius I:
 With him came Palaemonius, son of Olenian Lernus, of Lernus by repute, but his birth was from Hephaestus; and so he was crippled in his feet, but his bodily frame and his valour no one would dare to scorn.
Side note: Being a God did not shield one from Medusa. See: How did Athena keep from being turned into stone by her shield?