The thing is, everything Illidan is shown about the situation is through the lens of either the Burning Legion or the elder naaru sending him visions.
Both of whom have reasons to impart urgency by convincing him the Legion’s about to win. The demons because it shows that there’s no use in defying them, and the naaru because it shows that he needs to act now (naturally in the manner they want him to) or else.
Everything in the Illidan novel is filtered through either his own very skewed worldview or that of the agencies trying to sway him to their side.
It’s even worth noting that during Legion, it’s stated that if Sargeras isn’t stopped on Azeroth, he’ll use his victory there as the impetus to ramp up the Legion’s advance across the stars and start a new Crusade. Well, it wouldn’t be much of a new Crusade if it’s just having his demons keep doing what they were already doing to a last handful of planets they hadn’t gotten around to yet.