I would take a look at the Jewish Encyclopedia, which probably gives a more balanced treatment. Not all Jews taught a doctrine of original sin, but some did. Judaism was and still is an incredibly diverse phenomenon. It would be somewhat like grouping Anglo-Catholics, Lutherans, Presbyterians, Anabaptists, Baptists, Pentecostals, Unitarian Universalists, Mormons etc. all under the label “Protestant.” It’s not completely meaningless, but you can’t treat them as if they were a homogeneous group.
Second, I think you need to re-read the article (or correct me). I only skimmed the article, but I didn’t see Rabbi Singer allege anywhere that idea of original sin was unknown until the fourth century. He even lays the blame for the doctrine at the foot of St. Paul, saying that Paul twisted the Jewish scriptures to invent the doctrine of original sin. If you didn’t get your idea from there, whatever the other source with is wrong. Paul definitely does teach a doctrine of original sin, and so do the other pre-fourth-century Christian fathers.
Keep in mind, like others said, that there are different understandings of “original sin.” By the frequent use of the word “missionaries” in his article, I get the idea that the rabbi’s contact with Christianity is mostly with Protestant missionaries, who are focused on proselytizing Jews. If you had a lot of contact with Baptists asking, “do you know if you died right now, you would go to heaven,” and stuffing Chick tracts in your face, I can see how you would get a distorted view of Christianity. I would definitely have a bone to pick with the way the rabbi interpreted Paul.