The state of Michigan's insurance regulator notes that under Michigan law, if you are fifty percent (50%) or more at fault in an accident, you can be sued for up to $500 in damages to another person’s car that are not covered by collision insurance. Since the other party was completely uninsured they obviously did not have collision coverage.
On the other hand, they also state that if you are uninsured you may be held liable for all damages that result from an accident while uninsured, including your own. But this is likely meant for an uninsured driver that was at fault in an accident.
Since their language is unclear on the issue, to find out if an uninsured motorist not at-fault in an accident can sue you for the mini-tort amount of $500 or not I would suggest contacting either your agent to see how they understand the laws and/or a lawyer well versed in this type of MI law.
If you do not get a clear answer from your agent or a lawyer you may want to contact directly the Office of Financial and Insurance Services (OFIS) toll free (877) 999-6442. This state agency is the insurance regulatory body for Michigan.