The Arathi empire on Azeroth splintered more then a millenia ago . What is the Arathi empire the Arathi in war within are talking about ? None of them lived long enough to have seen it.
They are Arathi, they have founded an empire on the other side of the world, it’s their Arathi Empire. Imagine an East Rome and West Rome thing, just with no communication between them.
Apart from that… They have quite a few elves. Enough to breed elvish traits into the whole population. So why wouldn’t some have been long-lived enough to have seen it?
We will see them after the world soul saga I think. Averloren might be another continent on the other side of Azeroth.