Where did you learn this? It has not been in any article I ever found about the history of Rhode Island Reds.RIR have been, and may still be, used for cockfighting, so they have been bred to be pretty aggressive.
As a practical matter, any chicken breed produced by commercial hatcheries is going to be a very poor choice for cockfighting. The big hatcheries have many roosters in one pen with large numbers of hens. If they kill each other, there will not be enough roosters to mate with the hens, which means the hens will lay eggs that are not fertile, and then the hatchery cannot hatch enough chicks to sell. So it is very important for the hatchery to breed roosters that tolerate each other in the breeding flock.
Aggression toward people, and aggression toward other roosters, can be very different things.I have had some before, and if I do get some again, it will be when I have my huge chicken area done so I don't have to be as close to them.\
Several posters on this site have stated, repeatedly, that birds bred for cockfighting will NOT be aggressive toward people, because ones that are aggressive toward people will be culled. The point of cockfighting is for them to fight each other, not fight people.
On the other hand, I also see plenty of anecdotes on this forum about roosters that attack people but get along well enough with other roosters. I've had a few of those myself.