That quote is referrencing a rat study feeding them either 5% of their diet in casein or 20%. At 20% new tumors appeared regularly and at 5% no new tumors appeared. The study went back and forth in protein intake with the same population and had consistent results every time. And there's other evidence as well.
Before I watched that documentary I shot for Dr. Barry Sear's 30% calories from protein (almost all animal) and believed that was healthy. 30% is hell of a lot higher than 5%.
In 1985 Dr Essylstyn took 23 cardiac patients who were given know other medical options and put them on a plant based whole foods diet. Low fat, zero animal protein.
5 members dropped out. Of the other 18, only 1 had any further cardiac events in the next ten years. 12 are still alive today 27 years later.
Research of Dr. Esselstyn | The S File ? -- Health