How the Greyhawk isolationist elves are different from the Forgotten Realms isolationist elves and from the Dragonlance isolationist elves? I mean, you are saying this is one of the highlights of Greyhawk, but in my PoV this an already used to boringness trope.
Isolationist Elves are not unique to Greyhawk.
Fantasy Vikings are not unique to Greyhawk.
Fantasy Arabs are not unique to Greyhawk.
Fantasy Pirates are not unique to Greyhawk.
Former great empire fallen to a fraction of its former size and turned to fiend worship is not unique to Greyhawk.
They are all themes that are strongly present in the 1e boxed set though.
Use them as you like and it will strongly fit in Greyhawk.
The big uniques of Greyhawk are things like:
Scarlet Brotherhood political behind the scenes manipulator evil monks.
Iuz - evil demonic demigod open ruler of an expanding empire.
Wastri - demigod of a human ethnic racial purity and hatred of demihumanoids but also a swamp and frog aspect and lives in the world.
Zagyg - ascended mage demigod of chaos magic.