How in any other way can you sit on top of a circle? Otherwise he might aswell have just said he sits on the earth, not ' the circle of the earth(think). It's simple to see that Isaiah's source appears to have been the real God.
The top of a stool is a circle and you sit on that, don't you? Make that stool really big and you can stretch a canopy over it, set up tents, and watch grasshoppers jump on it, could you not?
Hmmm, moon is a disc, sun is a disc, Earth must be a disc. Not that hard to understand why the Earth would be called a circle instead of a sphere. I don't think it proves or disproves the validity of Isiah's God, just the weakness of a literal translation of the biblical text when it is obvious that it is wrong in specific description but perhaps not wrong in overall connotation.