The Sethite thesis holds that the “sons of God” (beney elohim) in Genesis 6 were the human descendants of Seth, and not the fallen Watchers. Among other problems, this view has to hold:
“The verb form (began) is third masculine singular. Since the word ‘adam, which is often rendered mankind…in modern translations, does not actually appear in the verse, the most natural reading would be that Seth began to call on the name of the Lord. If this is the case, then the Sethite view needs to extrapolate Seth’s faith to only men from that point on, since it is the sons of God who must be spiritually distinct from the daughters of mankind” (Kindle loc. 10377).
Heiser goes on to point out hat if you insert “humankind” into the text, you undermine the Sethite thesis, for then you have other human lineages calling on the name of the Lord.