Meaning of “Upon Your Belly You Shall Go” (Genesis 3:14)

You are correct. Satan and his angelic associates had already been cast out of heaven (Revelation 13:7-9).

However, the phrase on “your belly you shall go” can be understood as a further and metaphorical condemnation of Satan for seeking to oppose God’s plan and thus lead mankind astray, because animals who move in this way are proclaimed an abomination (Lev. 11:42). Similarly, in that same verse in Genesis 3, the phrase “dust you shall eat” in reference to the serpent is an idiom for enduring a humiliating defeat (Ps. 72:9; Is. 65:25; Mic. 7:17).