When everyone gets to a somewhat equal basis in college and on a college baseball field, the mental toughness of football can be far more helpful than any physical aspects in transitioning to the mental challenges of college/Milb baseball.
You may be right. My larger point is: has anyone ever looked at this systematically, beyond anecdotes about this kid or that? My guess is -- and it's only a guess -- that a far higher percentage of top-level baseball players focus solely on baseball in high school than either (1) the percentage who play multiple sports, or (2) the general assumptions about multi-sport athletes.
My quick glance at the first-team high-school All-Americans from California is not a large enough sample size to draw any conclusions. But I think it points in the correct direction.
As for the minors and majors, I really don't know. I mean, are all the Dominican and other LatAm kids playing other sports during their teen years? They sure as heck aren't playing football. What percentage of major leaguers played another sport in high school? I don't know the answer, but I know there are plenty of anecdotes on both sides (e.g., Giancarlo Stanton also played football and basketball in high school; Jason Heyward focused 100% on baseball from age 12 on, etc., etc.). And I think it would be interesting to know the answer.