Except that Shuyin and Lenne were from the Real Zanarkand, which got destroyed 1000 years ago. Since the the Fayth that dreamed up Dream Zanarkand were originally the citizens of Real Zanarkand, no doubt many of them knew Lenne and Shuyin personally.

So perhaps it was inevitable that the unconscious collective memories of the Fayth created Tidus 1000 years later. Although it may have been a case of the monkeys with typewriters.

Tidus seems to have none of Shuyin's memories though, even if he is a close physical copy of him.