Answer: That's not how it is. Yu yevon wasn't from dream zanarkand, he was from the real zanarkand, the zanarkand that was destroyed a thousand years ago. As far as we know, only Jecht and Tidus have every exited aeon-zanarkand, and tidus only got there with the help of his father.
I guess Jecht just got lucky. Who knows how many people try to get explore the oceans around aeon-zanarkand without making it. Perhaps sooner or later one of the inhabitants were bound to get lucky and hitched a ride with an earlier sin. Jecht just happened to be that person.
Aeon Zanarkand wouldn't necessarily have to be protected by anything. We've all seen the tech level of Spira. It's not really a lot to write home about. Being far out into the ocean, people might just not have had the means to travel there, and not the guts either, considering they knew Sin could crush them to bits anytime. Spira has an extremely large amount of water compared to land, so it'd also be like looking for a needle in a haystack. It wasn't until after aeon zanarkand was destroyed by sin that the al bhed gained access to an airship either
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Yes, I refer to dream zanarkand as aeon zanarkand because it is a better description of what it actually is. A gigantic aeon (or a myriad of smaller aeons) summoned forth by Yu Yevon, having physical properties within the world of Spira. It is not a dream like the dreams humans have at night.
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