Sargeras is stronger than anything on Azeroth.
He cleaves planets in half with his sword.
That’s what he would have done to Azeroth if the pantheon didn’t imprison him.
To make it clearer, yshaarj was the strongest old god in the black empire. And amanthul was the strongest Titan at that time. Amanthul ripped yshaarj from Azeroth like he was nothing.
Sargeras after his corruption was so strong he took on the whole pantheon at once and destroyed them safe for their souls which he sought after to turn them into dark titans.
The only thing sargeras was afraid of were the void lords and an awakened Azeroth.
The former can’t materialize in our reality so the only real threat to him was Azeroth itself which is why he wanted to destroy it