Yeah I find it rather hard to believe this. Legion going to end EXACTLY like Warlords of draenor, we were told that the Iron horde was the most dangerous threat we've ever faced thus far and they were defeated before 6.1.
What about the third war? The burning legion didn't even use their armies only their minions, and even then they managed to kill cenarius very easily, they destroyed the entirety of Lordaeron, they burnt Quel'thalas to ashes and they nearly destroyed The night elves and Orcs.
So with the biggest legion invasion ever, why are there only going to be demons on the Broken Isles? Why aren't there mass legions (Pun not intended) of demons everywhere on maybe say, Northrend or in Eastern kingdoms Kalimdor and Pandaria? Yeah they are going to attack in the prelaunch event, but so did the iron horde, how did that turn out? Neither did it make sense then, they had basically millions of orcs on the other side and they were just waiting for nothing, sending only 3 orcs at a time.
The only real villain we've ever had that actually acomplished something in WoW was Deathwing as he rekt the planet upon arrival and there were twilight cultists everywhere seeking to destroy the entire planet, now THAT felt like a threat! And this wasn't even the Old gods at their peak, there were no old gods at all to save them or C'thraxxi or anything. You might say that the Lich king felt like a threat aswell, but let's not kid ourselves, his armies lost at every turn, even if he killed us on the citadel, he still lost there so he didn't really acomplish anything. The so called Legion invasion is confined to a small isle and being defeated by some adventurers?
TL;DR: Biggest legion invasion ever? More like biggest pushover of an attempt at an invasion ever. The legion is not going to do any better than the Iron horde when it comes to winning. They are going to lose at every turn and be defeated before 7.1. The 3rd war felt like more of a threat than this, as did Cataclysm, because they actually caused damage, "The biggest invasion ever" is going to be confined to a small pathetic Isle.