Question concerning high elf area

[Volkyte]

Was just wondering. How is it possible that high elf area is not contaminated? I mean is straight behind stratholme and near naxxramas.
Should not plague cauldrons miasma reach there aswell? And in theory , original high elf zones being devastated by the scourge, would not cause an easy undead incursion in such area by sea?
I understand that game wise the available areas are those only, was just curious to discuss it from a lore standpoint being somewhat strange.

[Galendor]

There's a runestone on a border with Plaguelands that prevents spreading corruption. Besides, volunteers from the Alliance and Thalassian guards are defending its borders.
Regarding incursions by the sea - are we certain the Scourge has a decent fleet?

[Harkus]

The only reason Quel'thalas was destroyed by the Scourge is because they needed to revive Kel'thuzad with the Sunwell. An outpost like Alah'thalas not on the way to Silvermoon was simply not their priority.

[Volkyte]

There's a runestone on a border with Plaguelands that prevents spreading corruption. Besides, volunteers from the Alliance and Thalassian guards are defending its borders.
Regarding incursions by the sea - are we certain the Scourge has a decent fleet?

In wc3 they got. They manage to take Arthas from ice crown to Cordarone and back again.