> I have understood that with those really old recipes, the person recording them might at best have been in the same room. But probably was not a chef.

That's going too far. The person recording them might be the same person who is used to making the food, or might be taking literal dictation from that person.

Knowing how to make food isn't the same skill as knowing how to explain the process in a way that someone who isn't already trained to make the food can follow.