That’s a common misconception that’s been passed around the Internet so long that it’s become common knowledge. There’s a great (now deleted, but archived) writeup showing that the Wachowskis always intended humans to be batteries. The actual source of the “processors” idea came from a Neil Gaiman short story written to promote the movie at its release
Not sure which post you’re referring to, but from Our World in Data, pig farming generates more emissions than farming chickens. They are lower than beef, but that’s not saying much, since beef is an order of magnitude worse than most other foods. Pig farming also generates four times more emissions than soy.