He does line up really well. Not only is he symbolic to the founding of Thrall's good Horde, but he has a connection to the current Pandaren theme through Chen. While he is not technically a member of the Horde, from his perspective at least, I'm sure if they asked and he realized the kind of things that happen if he's not there, he would accept. The only thing I'd say about your conversation is that it's awfully convenient that Rexxar shows up right when they mention him. I'd much more expect that they decide on Rexxar and then go looking for him afterward, or the cutscene does a cut to Rexxar's wandering and they go find him (the latter would be a little weird if you were still standing next to Garrosh's remains after the cutscene ended).