He says that the relation of a supercomputer to man will be like the relation of a man to a mouse, rather than like the relation of Einstein to the rest of us; but what if it is like the relation of an elephant to a mouse?

There's nothing saying there can't be both entities equivalent to elephant to a mouse and equivalent to man to a mouse. In fact, we have supercomputers today that might loosely fit the elephant to mouse description. In any case, as mice, we don't have to worry about elephants nearly as much as men, and the existence of elephants might suggest men are around the corner.