Tidus didn’t know about summoners, or the war, since he didn’t exist in Spira for basically his whole life. He was in Dream Zanarkand.
It’s widely believed that the people in Dream Zanarkand are unique and different from the people of 1000 years ago. If the people were in fact real people from 1000 years ago, Dream Zanarkand would be stuck in loop. People die, and new people are born. These people are different from the people of 1000 years ago. Perhaps the initial population of Dream Zanarkand were that of the dead, but once they died, new people born. Tidus is only 17, so that’s over 980 years after the war, meaning he was never a real person (and not Shuyin either).
In those days, summoners were the defenders of Zanarkand.
In Spira the summoners prepare for the Final Aeon, if there was no sin why prepare.
The Final Aeon didn’t come into play until after Sin appeared. It was devised specifically to combat Sin.
Also why did not Tidus Know about The summoners or the war?
The dream Zanarkand was merely something to preserve the Zanarkand way of life. In the dream world, there was no war and no need for summoners to defend Zanarkand.