Did they earn income from that property, or any other source in Michigan (other than wages)? Michigan & Illinois have a reciprocal tax agreement for wages; wages earned in either IL or MI are taxed only by the state you reside in at the time. However, if you had MI tax withheld from your check you still must file a MI return. For more on MI's residency & taxation rules, see page 6 at https://www.michigan.gov/taxes/-/media/Project/Websites/taxes/Forms/2022/2022-IIT-Forms/BOOK_MI-1040... .
Though each state tends to read its residency rules differently, based on the facts given I would say your parents were full-year residents of IL. (Don't sue me or Intuit if MI says otherwise.) If they had MI tax withheld or MI non-wage income (income from MI property or businesses, gambling winnings in MI, etc.), they should file as a non-resident with MI. Otherwise, they only need to file in IL.
