It's an article about theoretical physics published in the 21st century, so it would be very surprising if it had not been posted on the arXiv before being publ

It's an article about theoretical physics published in the 21st century, so it would be very surprising if it had not been posted on the arXiv before being published in a journal. And sure enough: https://arxiv.org/abs/2102.06824 https://arxiv.org/abs/2102.06824