birdgoose@mekeor @zenmaya @boo_ @BeefGriller source code digger here. Looks like nil is defined as the integer 0 in the C sources https://cgit.git.savannah.gnu.org/cgit/emacs.git/tree/src/lisp.h#n619
BeefGriller is antifa@birdgoose Awesome find, and very interesting. Thank you for looking that up!