In addition to the omission of studies about plant based diets, it glosses over the main concern.

"Veganism is a philosophy and way of living which seeks to exclude—as far as is possible and practicable—all forms of exploitation of, and cruelty to, animals for food, clothing or any other purpose; and by extension, promotes the development and use of animal-free alternatives for the benefit of animals, humans and the environment. In dietary terms it denotes the practice of dispensing with all products derived wholly or partly from animals." https://www.vegansociety.com/go-vegan/definition-veganism

You could make a similar argument about slavery, the reason we abolished it has nothing to do with productivity estimations.