Hello from southern Wisconsin! All of you Chicagolanders are somewhat responsible for all the new road projects in WI recently.
Did he say he was from Chicagoland?
I guess Illinois = Chicago now.
He said northern Illinois, and if you're from northern Illinois, there's a 95% chance you're a Chicagolander.
But, having lived in Chicagoland, I'm not sure I've ever heard anyone living there refer to the area as "northern Illinois". They tend to say "Chicago", "Chicagoland", "the Chicago area", or even which end of the suburbs such as "north suburbs of Chicago".
Hearing the phrase "northern Illinois" without further context, I'm more likely to assume somewhere like Rockford, Sterling, Peru, or even Kankakee.