They had their seed in themselves after their own kind when created by the Lord. In Genesis 3:15 both the woman and the one being spoken to have a
"seed" https://biblehub.com/interlinear/genesis/3-15.htm zarah,zaraka from #2233
https://biblehub.com/hebrew/2233.htm
"The one being spoken to..." ,,the Serpent in the Garden in Genesis 3 though and those in Genesis 6 appear to not be the same (or in the same group). The one (the Serpent) is the Devil,the Serpent the Dragon
https://biblehub.com/interlinear/revelation/12-9.htm and
https://biblehub.com/interlinear/revelation/20-2.htm appears to be present from Genesis to Revelation while those who left their first estate and went after strange flesh are bound in chains until the judgment
https://biblehub.com/interlinear/jude/1-6.htm The one group(Gen.6) is bound in chains until judgment and the other group(Serpent and 3rd part of the angels)
is not. If the Serpent(spoken to in Gen.3:15) was in the same group as those in Gen. 6 he also would be in chains until Judgment.