I can't imagine that's the first time something like that has happened, haha. People can be extremely rude. Or perhaps he mistakenly only saw one finger so he was insulted and fired back
Although not the exact same situation, I had a similar happening. I was gasing up and grabbing a drink in town and I saw a rusted-beyond-belief TJ spitting and sputtering a few parking spaces over(Ohio plates. No inspection over there and I'm not too far from the state line). Originally I was slightly offended that someone would let their jeep fall into a state of disrepair like that. I pulled out making a left hand turn, and the guy and his jeep pulled out next me me making a right hand turn. Before he turns, he looks at me, gives me the biggest smile and waves at me. I had my cap from my drink in my hand so I did the best I could to wave back but I only managed to lift my middle finger and my pointer finger half way. To this day I really hope that guy didn't think I flipped him off. He made me realize that even though I wouldn't let my jeep fall into disrepair like that, we both love jeeps and he loves his enough to keep driving it even thought circumstances keep him from fixing it up into a dependable vehicle(whether it be money, time, or if he's just plain lazy).