It could just be plain ignorance. In-game characters are not all-knowing after all. It could also have to do with the fact that the approximate majority of the playable races are Titan's creations.
I'm quite sure though that they should know that Trolls, at least, are native of Azeroth and ancient as much as them, since their race was part of the Aqir, which its original reign was shattered to pieces thousand of years ago from the troll empires. The flow of time could have erased that memory though, since Mantid and co. are not immortal, nor someone would enjoy to document anything about the race that ruined your empire.