There is a difference, though:
The relation (as stated in the example) depends on a certain parameter or dimension in component 0 in the assembly, without actually relying on a certain object. So You may be able to replace the skeleton, and as long as the parameter is again present the assembly will update accordingly.
A simple constraint (in the assembly from the component to the skeleton) will break, if the skeleton is replaced or the component is used in a different assembly (where it will still require the original assembly for updating).
Using a copy of published geometry as local reference for the component constraint is a solution for he latter, but still needs the original skeleton.