This is true. There are multiple characters, both in DZ and Spira that say this.Originally Posted by Acid Raine
Tidus also uses this excuse throughout the beginning of the game until Yuna catches on and Auron returns, driving the attention away from Tidus.
It's best explained by Maxx Power's explanation and Sin's presence, but the difference is, is that DZ never had contact with Sin (except a few times where it may have actually interfered), so they would be more likely to feel the effects, depite Sin's distance (for the time) as they had not picked up any form of immunity. Being nothing but pure pyreflies themselves would also have an egregious effect.
The people in Spira weren't affected by Sin, until AFTER he attacked, which explains the tradition throughout early FFX saying that Sin was poisonous. This is likely untrue, because Sin is one giant hulking mass of pyreflies, and such a magnitude of them gathering would likely be to affect the pyreflies, who also somewhat steward your memories. But the people of Spira are also mostly flesh and blood, which means the effect of Sin is somewhat inhibited.