The word "circle" clearly resembles an understanding from Isaiah that he had insight to the shape of the planet. There really is no clear reason to deny the great possibility that a round earth was in his thoughts, yet at the same time it would be imprudent to rely on his statement as "ultimate proof" for Isaiah knowing of the spherical shape of the earth.
To conclude that the term "circle" can still mean "flat", such as in a "flat plate" is erroneous and unfounded thinking, because even if the world was flat yet a circular plate, how was Isaiah to know this? The "circle" terminology was probably his best way to describe it in Isaiah's thinking, imho.
Edited by Lysimachus, : No reason given.
™ Version 4.2