Not a good argument. That is just a standard paradox. The question as posed requires that god can't do something yet it is postulated that there is nothing that god can't do. Plus, if he can create and move planets then saying he can't make a rock in the gravity well of one of those planets that's too big for him to lift is nonsensical and is putting human limitations on a being that is not human, whether said being exists or not. Any arguments based on this, either for or against the existence of god, are basically worthless.

An atheism or agnosticism that depends for its justification on absolutist word games based on literalist interpretations of ancient mythology is built on ground just as shaky as that of the literalist believer.

The unliftable stone is much more real than god!

Modern theologians do a little tap-dance round this now, by saying that. "Gods power is the maximum logically possible." Which makes you wonder just how sad a human can get.

If such a stone existed it would require a great amount of force to keep it tethered. If such a force was gravity then perhaps the paradox created isn't that he can't lift the stone but it's own gravity also moves the object that it is now tethered to. Making it seem as though it is not being lifted but at the same time it is. Thereby he still created an un-liftable stone that by some definition isn't being lifted but at the very same time is.