The truth is that a number of Blacks had competed in MLB, but they were (falsely) called “Cubans.” Everyone in professional baseball was in on the scam, but that’s how they got away with using great Black players without admitting they were Black. Robinson's introduction to baseball and admission that he was Black was more of a marketing ploy by Dodger owner Branch Rickey than anything else.
This is to take nothing away from Robinson. He was a WWII vet, an officer, a great baseball player, and he was the first man willing to take the risks of admitting to being a Black man in a previously (supposedly) all-White sport.