The objector is saying that from “X is F qua R,” it follows that whatever has R is, unqualifiedly, F. Therefore, if Christ qua human is a creature, then everything human is, unqualifiedly, a creature. But we know that not everything human is, unqualifiedly, a creature, because Christ (who is human) is not unqualifiedly a creature, something Aquinas had argued for in an earlier article. Therefore, we ought not to accept the claim that Christ qua human is a creature.
