Your logic is perfect, but Creo is cheat-resistant
If I fix (Fix constraint) the object on the datums (if that's what you suggested), creo assumes that the object is the ground.
If I Pin the object using the datums, the not-well-constrained square appears between the icon and the name of the part and subparts on the model tree.
I will test this assembly on the bigger assembly, but I really think we are back to squat.