The proposed chain is: union is not inseparable from the parts; separation is possible; possible separation is within divine power; therefore separation can reach a last indivisible unit.
The objection targets the final step. The possibility of some division does not prove that every conceivable division has already occurred or that a sequence of division must terminate in a particular material atom.
Example: the capacity to halve any specified segment further does not itself imply the existence of a final unhalvable segment. That would require an additional argument.