PF configuration language that author considers to be “the clearest expression of firewall intent anyone has shipped” is 95% the configuration language of ipfilter, the initial solaris and bsd firewall software. Quite suspicious that the author doesn’t mention it, especially when pf started merely as a clone of the latter.

Considering the author is the founder of pfSense and has been contributing to BSD since 1995, I suspect he knows about the origins of pf.