Sargeras wasn't with the Titans when they fought the Old Gods on Azeroth.
There is no indication that Sargeras has had any contact with the Titans since he left.
Old Gods span the universe with the intention of conquering it and are not actually from this dimension.
"It is a small matter to control the mind of the weak... for I bear allegiance to powers untouched by time, unmoved by fate. No force on this world or beyond harbors the strength to bend our knee... not even the mighty Legion!" Of course, this quote could just be fanaticism.
A'dal says: "One in particular, a servant of the Old Gods known as Harbinger Skyriss, seeks to usher in his masters' vision of conquering all of the worlds in the universe."