That is not what your linked article says. It says there is not evidence of protein deficiency, and the deficiency of amino acids is overstated. Not that there is no deficiency.

And vegan/vegetarian health is really a 2nd order variable here. Vegans and vegetarians could have massive amino acid surpluses and it remains a fact that vegetable proteins lack useful amino acids that meat has. Maybe the vegetarians are eating lots of eggs. Maybe they are taking lots of supplements. Maybe they are actually eating meat despite calling themselves vegans and vegetarians. It doesn't matter. There really is no disputing the fact about the composition of meat/vegetable protein.