They're two sides of the same coin. They weren't Eredar before BC, because before BC, Eredar were 100ft tall primordial space demons who had always been demons and went around corrupting other races into demonic entities. There was no logical connection between the two. The WC2 manual establishes that the Draenei are natives, Gul'dan even refers to the orc's own planet as "the red world of the Draenei" hence it being called Draenor and talks about how they razed the fields the Draenei had been tending for thousands of years, it also establishes KJ as just one of the "daemonlords" from a different plane.
WC3 just shifted KJ to be part of a specific race, Eredar, who were still innate demons native to the Twisting Nether.