Meaning they've also experienced the humidity. I used to experience it every summer with a trip to the Miami valley of Ohio, about 100+ degrees outside and air you could eat. I've also been to Disneyworld in early August. That was thick.
I agree that many of them have probably been exposed to it somewhere. However, that doesn't really help them get used to playing a football game in it. In addition, as a transplanted Southerner now living in bone dry Phoenix, Arizona, I can tell you that even if you grew up used to the extremely high humidity, once you get used to lower humidity it's harder to deal with the higher humidity when you go back.