The author paints with broad brushstrokes in his intro, with several statements presented as observation but without support. He also claims to be a careful exegete in contrast to others. He then zeroes in on the situation with Cain with a remarkable literalness and lack of realistic imagination that he has accused others of. Go figure.
Why must the passage be interpreted to assume just four people? He seems to have little appreciation for storytelling, much less any expertise. Where does it say Cain had to find a wife? That is an issue read back into the story using a technique called eisegesis. The text clearly implies Cain already had a wife, and yes, she was almost certainly a sister. It is common scientific knowledge that inbreeding within a population becomes a problem only after several generations, and that is in the modern world. Genesis clearly depicts an antediluvian world with far fewer physical problems.
Really, the man seems to have no experience with the real world. Historically, the general practice has been to have children rather regularly over a long period of time, not wait until two are full grown, then have more. And where does the passage say how old Abel and Cain were at the time? They appear to be living independently.
Bottom line, I agree "Cain’s narrative suggests that Adam and Eve were not the only humans alive at the time," but per the passage they were all clearly descendants of Adam and Eve. His assumptions leading to his conclusions seem wildly unrealistic and unnecessary.
