Again you seem to ignore the fact that the slaves were grandfathered in
? I had brought up that very aspect of slavery in Illinois in an earlier post (#1348).
But ok let's explore it. As far as Illinois was concerned, and per your own cite of the clause, it was not slavery but "service by contract or indenture" and Illinois had surrendered oversight of it. The held person, when freed, was freed via terms of their contract or indenture, not by decree of an Illinois court (Illinois had banned slavery so wasn't issuing decrees of Emancipation). Illinois counties would record and file out-of-state decrees of emancipation, or file records of completion of indenture, just as it would file such papers for any other kind of contract.
We realize that if it can be sold that Illinois was legally practicing slavery -- rather than just documenting its status and meeting its obligations under the Fugitive Slave Law -- the hope is that folks will see the North as no less culpable than the South for maintaining slavery right up through the CW. Subtext: the war wasn't over slavery.
Of course that ignores all evidence of intent and scale. The practice of virtual slavery in the free Northern states cannot nearly be equated with the proactive, endemic and massive practice of chattel slavery in the South, the Confederacy literally founded on the precept of slavery.
That's not to excuse virtual slavery in the North, and no one has been. Tilting at windmills.