Why is your Rav wasting Shiur time to talk about other religions if he holds you should learn on Nittul?
The word Nittul is a twist off the word they called it, which means birth, whether or not he was born now.
Back-engineering a Minhag is a nice thing, until you use that to uproot it. Perhaps it is a good time for refflecting that Torah is Kedusha and we don’t want any part in unholy Torah.
I use the word Nittul as a Remez to this. Chazal say, Kol Hayesser Kinittul Dami, an extra growth on a vital organ is the same as a part missing. Since the birthday boy added, we equate that with taking off.
If it would be about Sakanna alone, it could have become a time for any specific activity instead of Batallah. A Tefillah, Dreidel, Seuda or even simply to learn at home. In fact, this Minhag might have been dangerous itself. It was used in debates to show that we engrain hate and mock them. It is the Yeshivos that dropped this along with any Hanhagah not explicit in Halachah Seforim, and Inyanim per se were and are frowned upon. This reminds me of the debate in Yeshiva about learning Mishnayos for a Niftar. The Chasidim were Taaning that if you don’t like Inyanim you should learn Ketzos for a Neshama.