The thing with manure is it could have x pounds of n-p-k and some trace minerals in it, but only a small percentage of it is available in the first year. Between the commercial fertilizer being available at the time of application and the manure availability in post years , it’s a very strong possibility that he benefited from both, thus giving him a great yield.
The added benefit of manure is the organic matter that is being applied, thereby improving soil health.
In my opinion, there is no magic solution to growing crops. Every farm is different and every farm needs to understand how their soil profile works to ensure they apply the nutrients required.