The answer is quite simple. Horde and Alliance both are not active on the Dragonisles. They allowed organisations to form the Dragonscale Expedition at the beginning of the expansion. Other than that, they weren’t really involved. The difference in 10.2 however is, that (as Alexstraza mentions) she send envoys to both factions and all races of Azeroth to ask for aid in that very conflict with Fyrakk but that something blocks their calls for Aid and that’s why they appear later after destroying the “beacons” that prevent them from entering the Emerald Dream, why they know what is going on and what will happen if they fail to stop Fyrakk.
See it in comparison like when Gandalf in the Hobbit appears to inform all of the Orcs coming to the Mountain and how they have to fight together. Same story trope, basically. Just a different perspective. We just never have seen (literally) those envoys do their work.
And I gave explanation to how Horde and Alliance know from the Dragonflights what’s going to happen, which know it very likely from the Green Dragonflight and Viranoth spoiling Fyrakks plan.
I asked how Fyrakk knows what he is doing will have the effect he think it does. From all ive seen he isent a planner nor knowledgeable about the ways of magic unlike Neltharion or even Iridikron (could they studdy while being imprisoned?). Hes written like a fighter that happens to be infused with fire.
You only listed events that happened, explaining nothing.
As far as I know he spoke to the Druids of the Flame(?) beforehand, so they could have possibly told him the result of corrupting the World Tree with Shadowflame. Maybe it was the essence of the Shadowflame itself, whispering to him.
I am sure there are enough ways how he could know about his doing. But after all, he is also to some degree insane. So yeah.