Whitfield Diffie is also an Emacs user. I once sat behind him at an awards ceremony and was able to observe him hacking on Emacs lisp during the event. That was the highlight of my month.
Yes, that's exactly what he was doing when one time I approached him to sign my copy of his book!
What does hacking ON Emacs lisp mean? Does it mean working on Emacs's lisp interpreter's source? Or on some project written in Emacs lisp?
For me it means writing an .el extension.
Yes. After the event was over, I asked him what he had been working on, it was some code to identify what sort of buffer he had just opened (secure v. insecure, I don't remember the details)
I would call that hacking in Elisp, not on Elisp, which is why I asked.