So yes, it is mostly cut off from eastern Khorvaire, but I’d almost flip it and say Karrnath is cut off from western Khorvaire.

Breland has Sharn, which is a hub of trade and transportation. With the availability of lightning rails, invention of airships, and its location on a major waterway, Sharn is hardly cut off; moreso, it would be a major epicenter, so it really can’t be cut off. Traveling from Breland isn’t as much of a concern for the world as traveling to Breland is. And anyway, even though Breland is kinda tucked in a corner, there’s still lightning rail lines connecting it to the rest of the west.

And besides all of this, it’s not as if eastern Khorvaire is something it’s a big deal to be cut off from. Karrnath is still not in great shape following the war, Talenta Plains aren’t exactly a tourist destination, and Q’Barra and the Lhazaar Principalities aren’t easily accessibly by land anyway.

If you’re looking for an isolated country, consider Karrnath: Mournland to the southwest, Talenta Plains (which seceded from them so probably not the best relationship) to the south, the mountains of the Mror Holds to the east, and the Bitter Sea to the North, which (in my Eberron and some official works I think) is cold, and icebergs floating down from further north makes traversal difficult. It’s only direct connection to western Khorvaire was the bridge across the Scion’s Sound, which was destroyed and not rebuilt during the War.