Back in the day, many private school teachers did not have teaching degrees. They often spent time in college (usually liberal arts schools focused on a content area). That was part of the appeal. I know people who teach in private schools who have PhD in English but no Education degree and they are great, involved teachers. The question of whether a person with a BA in Education or English is a better teacher is a bit of a herring. I have seen plenty of bad teachers with Ed degrees and plenty of bad teachers with a content degree. JMHO.
The fact that he dropped out of Cooper Union is/was a concern but we have no idea when he went to NYU---if it was at the time he was teaching, they may have had an agreement that he needed to get the BA/BS soon. Everything around this man speaks to some sort of powerful personality that many take him at face value while he then goes on to exploit children, relationships, etc.
I can understand the lack of a teaching degree, per se. Public schools I attended did the same, e.g. math and science teachers at our schools who had PhDs, but no teaching degree. They were excellent teachers.
The problem with JE is that he had no degree at all. Zero. Just attended college for a brief period. That seems unacceptable, JMO. Thus far, I've seen no proof he was a mathematical genius. He worked in the finance sector where just being a clever con artist is enough to make you successful. JMO