ok here's the thing, dream zanarkand did exist in reality 1000 years ago before being destroyed by sin, but tidus wasn't in the real version of the dream zanarkand.

then the fayth dreamt up zanarkand to keep it alive.

1,000 years into the dream zanarkand tidus exists but then sin transports him out of dream zanarkand and into spira, where zanarkand is actually a ruin.

the attack in zanarkand by sin that tidus experiences is not the original attack that caused zanarkand to be a ruin in the first place.

for those of u that hav played x-2, shuyin and lenne did exist in the real zanarkand 1,000 years ago before it was destroyed in the initial attack by sin. as did (SPOILER)maechen