Apparently Sisko was a student of history. What he said about Las Vegas of that era was more or less accurate. He didn't want to play pretend and pretend that segregation didn't exist at that time. That didn't seem unreasonable."Badda Bing, Badda Boom"; Sisko gets weird about the Las Vegas holosuite sim. I always appreciated the subtle ways Avery Brooks worked in references to Africa, between the wall art and his civilian clothing, but that scene was a little obvious, and (in terms of reality) weird. I still don't think a character who lived in the 24th century, where so much history had been obliviated by all of the stuff humans had to absorb -- paradise on Earth, technology, Vulcans, Klingons, etc, etc -- would be that passionate about a social situation from several centuries prior.
It may be questionable whether a 24th century person would get so upset about it. But Sisko was a passionate person. He seemed to have a passion for African culture. He was also passionate about creole cuisine. And he had a love for baseball and baseball history even though professional baseball went extinct centuries ago. A 24th century man having a passion for baseball. Odd or not.
Sisko's wording may have been out of the ordinary, but his pointing out that particular fact of Vegas history seemed reasonable.