Lol, you're trying REEEAAAALLY hard to push your agenda and have resulted to grasping at straws now.
They took over an entire planet, distorted it into Fel and ruins, turned its people into powerful soldiers and their Titan into a battery. It's conquered by every sense of the word.
You can say Sargeras manipulated 2 of the 3 leaders with sweet talk, but the end result was a conquered world and a central hub for the Burning Legion.