Sorry if it wasn't clear. I gave an abbreviated form of the accepted answer.

First, "a rock too big for God to lift" is a logical impossibility; it's exactly the same thing as saying A=1, B=2, and A=B, and then asserting you've proven something about A and B. Defining a god as omnipotent, and defining a rock as too big for omnipotent beings to lift, is nonsense.

Second, logical possibility and metaphysical possibility are not the same thing. The fact that they are not identical to each other, and therefore do not necessarily directly correspond, is widely accepted by philosophers. In particular, it is agreed that whether you can state or imagine a logical (im)possibility says nothing about the existence or truth of a corresponding metaphysical (im)possibility.

To summarize, the statement is first of all a logical impossibility; that is, manifestly untrue even on its own "turf", that of logic. Second of all, the question of whether such a rock (or such a god!) can actually exist is a question about metaphysical impossibility, and no statement of logical possibility can directly prove something bout metaphysical possibility. This is a type of error that is normally called a "category error"; where a thing (or a property) of one kind is mistakenly reasoned about as though it were of different kind that does not apply.

In this case, another way of putting might be this: if an omnipotent god actually exists, they can lift all possible or actual rocks that can be created. If the category of rocks they can lift includes all possible or actual rocks, well, no other kind of rock exists or can exist. Whether we can propose nonsense like an omnipotent god lifting rocks that can never exist is unrelated.