On the surface this statement seems implausible, but there is research showing that protein intake immediately before or after a workout can reduce the increase in testosterone that occurs after when you train with high intensity.
There are also studies that showed that high protein diets lower resting levels of testosterone, while other studies have shown that protein intake can increase the availability of testosterone.
These seeming contradictory studies may be explained by a theory that protein could cause an increase in total testosterone while also lowering circulating levels of testosterone. The theory postulates that protein intake increases the sensitivity of testosterone receptors on cells so that the increased testosterone is being pulled into the cells which decreases circulating levels of testosterone!
A 2008 study showed an increase in the production of testosterone receptors within exercised muscles in response to whey protein supplementation done at 1 hour and 48 hours post-exercise. More receptors mean more testosterone is taken up by the cells!
The 2008 study results were supported by a 2015 study using rats which showed that whey protein supplementation immediately after “training” resulted in a 90.5% increase in intracellular testosterone. Rats were forced to exercise vigorously and then given 500 milligrams of whey protein (this is equivalent to 19 grams of whey protein in a human which is a typical post-exercise dose).
Given that many other studies have shown increases in muscle growth with whey protein intake post strength training it is pretty safe to assume that whey protein intake after strength training will improve muscle growth and recovery.