It is probably not intended to be a chisel in the woodworking sense, but rather a scraper/cleaner. Old tires cook the rubber to the rim sometimes, and that would be usefull to someone fixing tires to scrape the rubber off. The fact it won't hold a proper edge wouldn't matter, it's being used against steel anyhow, it would simlpy be reground a lot....
Making it from a tire tool would make sense if the tire tool had a broken end, which happens to them sometimes when wedged too hard and pried....
Only a theory of course, the world may never know....