I know from sea fishing.... Once you kill the fish, you need to gut it soon ( or ice the entire thing down).
If you don't, and let the now dead animal cool down slowly and naturally, then the worms in the intestine get unhappy that their host is cooling down, and then start to migrate from the intestines into the flesh.
Maybe he did the same sort of thing with his chickens?
If your chicken has a "normal" load of "normal" parasites, and you process properly (kill and then quickly remove all of the insides) the meat is perfect.
" normal" here means those parasites that are common and not greatly harmful as long as their numbers remain at a moderate level.
As an esteemed professor at Tulane Medical School used to say "Worms, there is nothing wrong with worms, as long as you have the right kind of worms, and not too many worms. I, myself, have worms."