This talks about “true men”. I see “true men” as a physically human body, as produced by evolution, with a human soul directly produced by God.
Start with a population of almost-humans who are physically like us, but unsouled. Call then ‘huma’ because they are not quite human. God gives two huma souls, so they become human, in both body and soul. If you like, the second human can be created from the first’s rib.
Those two humans, Adam and Eve, have children, to whom God also gives souls.
To solve the problem of human genetic diversity allow Seth and his siblings to marry into the huma population around them. They are biologically compatible, with compatible DNA, so those marriages will be fertile and any children will be descended from both Adam and Eve as grandparents. God can give those children souls.
Hence there is a single human couple who are ancestors of all living humans, while the huma provide the required genetic diversity we see in modern populations. Eventually all huma die out as ensouled humans spread; only one ensouled human is needed in a couple to allow all children of that couple to be themselves ensouled.
To me that scenario seems to satisfy biology: a minimum population of 10,000 breeding pairs, and descent from a single couple. By treating souls separately from DNA and physical bodies the apparent paradox can be resolved.