It's the other gods. When the proto-Hebrews created the Genesis story, they were surrounded by people with lots of gods. Like most folks back then, they accepted the validity of the existence of those gods, but assumed their God was the big kid on the block. Indeed, through much of the OT, the assumption is that other gods do exist (Baal, the gods of the Egyptians) but that the Israelite God is more powerful than any of them. I'm not sure that the concept of God as a singular entity in that domain really took hold until a couple of centuries after Christ.
Of a piece with "no other gods before me".
So an omniscient God speaks of a none existent we.
I wonder how the literalists explain it.
Tutankhamen did attempt to push montheism onto Egypt.