nope, it's the island in the lake.
The ability to sail to it was removed before Lake Austin ever existed (in fact, before Trammel existed, IIRC), so any Fel rune would be one that someone cross-sharded. Any Trammel rune is also from another shard, but from where someone used the old moonstone system to jump from the Fel Island to its Trammel counterpart, then marked.
I've always wanted to go there, myself...