Some rabbis have suggested that the Genesis 5:3 statement that Seth was in "Adam's likeness: hints at Genesis 6's world of weird angel/hybrid creatures.
Adam himself is TECHNICALLY not "just a human," being born of dirt and spirit.
ויולד בדמותו כצלמו, “he begot a son in his likeness in his image;” the emphasis on this is to show us that anything he begot during the previous 129 years were only creatures that did not reflect his likeness or image, i.e. disembodied spirits, mostly מזיקים, injurious, destructive spirits. (Compare 3,20)
Chizkuni on Genesis 5:3
BUT.
Other rabbis in the Midrash have said that being "in Adam's likeness" meant that he was… born circumcised.
He was righteous (Gen. 6:9). This suggests that he was one of the seven men born circumcised. Adam and his son Seth were born circumcised, as it is written: He begot a son in his own likeness after his image, and he called him Seth (Gen. 5:3).
Midrash Tanchuma on Genesis 5:3
Which is, by the way, NOT ANY WEIRDER THAN ANGEL/HUMAN MONSTER BABIES.
So, always take commentary with a grain of salt. Even mine.