Comparing this with 1 Kings 5:13-14, it would seem that a modified service of forced labor for one-third of each year was not regarded as reducing those who were subject to it to the condition of bondmen.
Related Commentaries of 1 Kings 9:22
1 Kings 9:22
22 But of the children of Israel did Solomon make no bondmen: but they were men of war, and his servants, and his princes, and his captains, and rulers of his chariots, and his horsemen.