There's more to the story. It's not like these kids did not know what a baseball bat was and became good in a three year span. Anderson played thru middle school. Recalls his first LL HR as his best baseball memory. Murray is the son of a former Major Leaguer.
Either way, these were athletic kids growing up who were outside with neighbors, friends, older siblings playing whichever sport was in season. All day, everyday. Some just never got into organized baseball. We have a kid like this in our town. Played stickball with all the other kids, just never played baseball as it interfered too much with basketball and the start of football. He's on the HS team now and he's going very well.
I actually think baseball players are the best athletes. They can't jump or cut like some of the guys in other sports but if you take a baseball player and put him in any other sport he will compete. Ask a basketball player to grab an AB against 80 mph and he has almost no shot. The footwork and slickness required from baseball is unmatched in any sport. I've seen some NBA/NFL guys try to throw a ball before. It's not pretty.
Baseball is a skills game primarily with the hands that has some athletic aspects but far outweighed by skill. Clearly you are correct those skills would not be present for more than a subset of the football and basketball players and watching people try to handle a baseball that are unskilled is comical.
But to say John Kruk or Jim Thome is an athlete compared to almost anyone in the NBA and most of the NFL minus some OL and DT isn't correct. Every MLB team has 5 Kruk types sprinkled among the P C and corner infielders. Hell one of the arguments for the DH is pitchers get hurt running the bases. When there is an argument being made that you need to limit the participation of players because they cannot run in a straight line with nobody in their way - you don't have an athlete. They have a strong lower body and lightening in their shoulder and elbow. A rare skill to be sure - but not necessary accompanied by athleticism.
I would bet if you had just the guys going to P5 football schools for 4 HS classes as QB, WR, RB and DB that you could generate 50 MLB players and a dozen All Stars. It would never work the other way around. You couldn't put the baseball recruits in football and come out with enough NFL caliber players at the back end.
Bo Jackson and Dion Sanders proved you could be good to great at baseball while being great football players. I think there are many more of them out there than we realize. Don't forget Frank Thomas was a TE as much or more than he was a 1B when he got to Auburn.
Guys like Beckham or Julio Jones jump to mind. Could Gronk have been Judge before Judge?
Michael Jordan was less of a joke at baseball than people give him credit for. Klay Thompson has a MLB brother who is not physical specimen he is. There would be more guys than you think in the NBA but maybe even more of the guards in the 6' to 6'5" range that don't make it - of which there are 100's every year that would have probably been killer baseball players.
I will leave you with this....Tim Tebow is in AAA and did not embarrass himself in spring training. He took a decade off from baseball. Had he gone baseball rather than football immediately after Florida it is likely he would have become a MLB level player. He probably won't now - even if the Mets bring him up to try to sell tickets in Sept.
As with the Murray thread - Milb is a problem for attracting this demographic. NBA and NFL pay big early and there isn't a single bus ride from Abilene to Lubbock to be had. So they will probably continue to spit out 100's of guys every year that might have had other results with a few years of bus rides. It is too bad for baseball that this is so.