See Job 38:
4 “ Where were you when I laid the foundations of the earth? Tell Me, if you have understanding.5 Who determined its measurements? Surely you know! Or who stretched the line upon it?.
6 To what were its foundations fastened? Or who laid its cornerstone,.
I don’t see what you’re seeing there, I guess. Nothing in those passages supports a flat earth. Quite the opposite. In fact the question format of the phrase suggests a nullification of the premise. By asking “to what were it’s foundations fastened” the speaker is saying that there is no such fastening. The earth is supported by nothing— it is it’s own foundation. Which... is true.