It's not so much that God cannot create the object, it's that the object is logically nonsensical. Can God draw a square circle, for example? Can he make a 3-dimensional square? Well, "square circle" is nonsense, as is "3-dimensional square (of course, there's the cube, the 3-dimensional analogue, but it's a cube, not a 3-D square), because squares are defined to be 2-dimensional, and circles are defined insofar that they cannot be square.
In the same way, "an object which God cannot lift" is nonsensical, because God - on account of his omnipotence - can lift all objects.
The real issue here is how you define omnipotence, and whether you include, in that definition, the ability to make and do things which make no logical sense.