Indeed Pirenzepine[1] is a muscarinic receptor antagonists. “The muscarinic receptor is a protein involved in the transmission of signals through certain parts of the nervous system, and muscarinic receptor antagonists work to prevent this transmission from occurring.“[2]
I could imagine that a let’s say relaxed nervous system recovers better.
Yes, he did take an oral dose, I believe it was higher than recommended dose, so some risk involved. I don't really see any studies on oral administration, but the new cream showed systemic relief.
As in taking it orally for his SFN? That sounds equally fascinating as it sounds unlikely to me so I'd def love to read a bit more about it!