Even small-scale mining operations can yield substantial amounts of gold, especially with such high-grade.
I know you don't understand the concept of sample concentration but, the grade of the concentrate is not important. Because it was concentrated 800-fold from the original sample, the grade of the original sample was around 0.14 oz/ton- that's the important number. Not particularly high grade, and far from "Bonanza grade".
Plus....one sample from a large pile of rock is not defensible statistically.