Sorry, Dr. Feser is absolutely correct. It is you who are inaccurately expressing the teaching of the Magisterium in this matter, as is evident from the following quotation taken from a peer reviewed entry in the New Catholic Encyclopedia Supplement.

"Finally, the CATECHISM OF THE CATHOLIC CHURCH speaks repeatedly of Adam and Eve as a single mating couple who “committed a personal sin” (CCC, sec. 404). JOHN PAUL II's Apostolic Constitution FIDEI DEPOSITUM calls the Catechism “a statement of the Church's faith and of catholic doctrine” (CCC [1994] 1997, 5)."

Taken from "Monogenism and Polygenism" by Dennis Bonnette
New Catholic Encyclopedia Supplement 2012-2013: Ethics and Philosophy, page 1013.

Aw, thanks :)

Though come to think of it the existence of Neanderthals does have interesting implications under Feser's hypothesis. It depends on what we think souls imply; Neanderthals apparently used advanced tools, including making dugout boats and sailing the Mediterranean; they buried their dead, and genetically speaking it appears they had language like us. If we infer from these facts that they had souls then that would mean that Adam and Eve existed in the ancestral species that led to both modern humans and Neanderthals (so, about 350,000 - 400,000 years ago at the latest) (Wikipedia suggests this would have been Homo heidelbergensis).

If we don't infer from these facts that the Neanderthals had souls (I was going to refer to the lack of Neanderthal art, but a quick Googling to check shows even that stereotype may be false, they found Neanderthal-inhabited caves with strange abstract art-like markings in 2012), then Adam and Eve could have been modern humans, and the interbreeding events with Neanderthals would have involved ensouled humans mating with soulless "sub-humans", even if it wasn't the specific large-scale interbreeding event that Feser's hypothesis involves.

I really appreciate your thoughtful response. I teach Biology in a Catholic High School and am always looking for a solid explanation for these types of questions.

They wouldn't fit any criteria at all. Feser's sub-humans would have constituted the majority of our genetic ancestry - that's the point of the hypothesis, to explain why today's humanity are the genetic descendants of many thousand people, not two. And Neanderthal genes make up a tiny proportion of the gene pool of a few human populations, and are thought to be the result of very rare interbreeding events (otherwise the proportion would be higher). When that interbreeding took place (apparently 50,000 years ago) also doesn't coincide with when population bottlenecks are thought to have occurred either (not that those population bottlenecks need to coincide with Adam and Eve's appearance under Feser's hypothesis), and it's much, much later than the appearance of traits we associate with having a soul, like painting, music, jewelry, or honoring the dead.