I question the usefulness of the study based on the fact it is just doing estimations of the input but not looking at actual blood levels of proteins and nutrients. There are other studies that I've seen the past that measure protein levels in the blood which give a surprising amount of support to a vegetarian diet actually supporting higher protein levels. Now there has been a maintained concern over getting all the amino acids and if you can't intake certain plant sources then you might need to use vitamins to supplement for certain things.