And...thinking a bit further...we all know who has what types of muscle fiber, and how training specificity and body type influences all that. I guess you just weren't thinking clearly.
That gets back to what is common sense 101. If you're destined to be skinny, you might gain some weight / gain some muscle, but, you'll still be disposed to being thin. That's just how it is. Fat folks can get thinner / gain muscle / lose weight, but, like it or not, they still have a disposition to go a certain.
Do some more research. I think you'll find fiber types are influenced more by training specificity, and sport chosen, than you originally stated.
In my 31 years of observation, I've seen academia make a pile of what was clearly mistakes. E.g., AAS have no influence on sports performance (right, SIC). AAS are very dangerous (not really). Lactic acid causes soreness (no, it's micro tears). Low HDL causes you to drop dead (nope..studies say just the opposite in active populations). So on and so forth. Ever see a movie called "Reefer Madness?" Academia would do well to be more honest, and...more humble. The media would do better to stop with sensationalism and to be more broad in their research.