The amount of people failing their lore - and then posting with conviction - is too damn high...
The main reason titans won the war against the old gods is that the old gods were also warring between each other - and they have never prepared their kingdoms for such a planetary defense, since they have been warring between each other for millenia instead of cooperatively building defenses and armies.
But when comparing raw power, the Old Gods are vastly more powerful than the Titans - they just lack the military organization and cooperation to successfully win a planetary war, while the Titans are a very disciplined very militaristic organization.
Just take a look at the lore sources - sargeras got wounded by a time traveling orc wielding a wooden axe, and he also got easily corrupted by the nathrezim.
Titans are strong - but they are incredibly susceptible to corruption and they are also rather easy to wound, even by mortal hands.