I gave you a link to a message in which this very assertion is being made, and another link to a book which shows such a law was actually passed during Lincoln's tenure in the Illinois legislature.
I did do a bit of digging and found that Lincoln was one of five who voted against one discriminatory law, but the message I linked for you asserts he did vote for other laws of a similar nature.
But don't seize on just the Lincoln bit, also address the Illinois white citizens rounding up Free blacks and selling them into slavery in the South. Acknowledge that Illinois, as a state, was just as racists and hateful as people try to make the Southern states out to be.
You assume that I see the antebellum period as a North/South problem - you could not be more wrong! I see it as a slavers/abolitionists problem; and by the 1830's-40's the Demorats (spelled the way I wish to) became the Party of slavery. If slaver laws passed in Illinois during the time of Lincoln, it was mainly because of Demorats (maybe with some pro-slave Whigs at the time). For me, this North/South thing was incidental/providential.
We need to remember that slavery in Illinois had long been legal under French and British rule, and even when Illinois became a US territory and slavery supposedly outlawed (1787), the Federal law was not enforced and slavery continued in various forms.
In 1824, Illinois voters narrowly defeated a proposal to make slavery lawful in Illinois.
In 1825, the Illinois state supreme court finally began to enforce anti-slavery laws, and in 1848 the new Illinois constitution specifically outlawed slavery.
Nevertheless, Illinois retained some of the strongest "Black Codes" in the country, forbidding blacks from outside the state staying more than ten days without paying a $50 fine, or facing deportation.
But the key events our FRiend DiogenesLamp remarks on:
Bottom line: like some other Northern states in 1860, Illinois had a strong pro-slavery minority, and plenty of ruffians eager to enforce the South's Fugitive Slave Law by capturing & returning black people, regardless of their actual legal status.