That seems like a very likely possibility. I doubt most people would be agile enough to poo on a windshield so it's likely someone had the poo in a bag. Since most people don't carry around their own poo in bags, I'm betting it probably came from a large dog as most dog owners have been known to carry around their dog's poo in bags. Since the X owner still has a poo sample, a DNA test would be able to confirm this.
One of my neighbors used to have a problem with cars parking in front of her house. She suggested more than once that she could place dog poo on the windshield to deter them. I don't know if she ever followed through. She does not live in Chicago but perhaps some people there have had the same idea.
I love how we're so good at speculating on this forum that we're even speculating how the person committed the crime, and if it was even a human!