It’s a bit of an odd answer, really, and it all depends on which War of the Ancients Blizzard considers canon these days, because even Chronicles has been vague. There are two versions of War of the Ancients, the original and the Knaak version shown via the novels he wrote.
In the Knaak canon, the War of the Ancients lasted for months at most, and Illidan didn’t ever really develop demon hunter skills before the Legion was defeated. He did get his special sight, and the power boost, and saw the truth of how vast and powerful the Legion was, but there was no time to teach demon hunters, and he didn’t even get his famous Warglaives in the novel before being imprisoned.
In the original canon, the War of the Ancients lasted for years, possibly decades. Illidan, like in the novels, betrayed the kaldorei at one point, joined up with Azshara, and got his fancy eyes. Once he had the info he needed, he rejoined the kaldorei to reveal he was a double agent. He then was able to train some select students in demon hunter methods similar to his own, before he was imprisoned after the war ended. These select demon hunters continued to follow his teachings, killing demons where they found them, as outcasts. This is where Altruis the Sufferer came from, as well as the female demon hunter we saw teaching recruits in Black Temple, and the demon hunter in Blasted Lands.
Blizzard’s been very vague regarding which of these is actually canon, referencing both of them in equal measure. Demon hunters even existing in canon before Legion doesn’t make any sense in the Knaak version, but we have an entire instance referencing the events of the trilogy (the Dragon Soul).
Edit: I… read the question wrong. Can sum it up as a lore plot hole in terms of actually getting there, though if the original War of the Ancients canon is used and there were demon hunters before Outland, then that might also explain it. Demon hunters would probably have ways of getting there.
