It's not my opinion, the Negro League is something that became proud part of American history after initially being seen as a "lesser" league that was in place obviously because of segregation.
The men that played in it are a massive part of sports history and as the other poster noted they played with honor and pride and used that platform to display their skills and fight for racial justice. Many would go on to be great baseball players and true activists that are proud of their time in that league. Every year at different points through the MLB season, teams and the league will pay respects to the Negro League, you'd see black players of today talk about it and players from the past who played the the league talk about it in this way. There was actually a while where the jerseys and hats were a big deal in pop culture.
They are referring to something that existed, and is an actual part of history. To be frank, you don't quite get it, and I wouldn't expect you to since you aren't American. You can't pick up every bit of information you have in regards to a matter from the internet and really understand the nuance and intricacies of what happened in the past and what it means today in something as niche as sports, baseball, race, and Americana. Doesn't make any sense to refer to something that existed in the past by a different name today. There is no value in that. The word Negro is pretty darn uncommon and just because it is used in the name of the Negro League or a few institutions does not mean it is a term that is used often or by most people at all.
Usually you'll hear the term used in context of some historical matter or something to do with education (United Negro College Fund) the writings of Frederick Douglass for example..etc., or religion "Negro Spirituals" or some similar matter, p4p some of the most stirring music ever made, btw.