Under your theory, people without fat on them cannot gain muscle. There is some truth to that, but not in the way you think.
Building muscle requires 3 things: Stress damage on muscle tissue, adequate energy source/nutritional intake for rebuilding said muscle and the correct hormones/genetics/body-composition to encourage it.
Fat barely plays into it, except as an additional energy source if necessary. Body builders generally gain fat and muscle at the same time, then lose the fat afterwards because most people's bodies resist adding muscle when that extra energy source isn't available.
Losing fat generally also generally means losing muscle mass unless you had comparatively little to begin with.
You don't have reality on your side in this.