An IDP is merely a translation document. Unless you were going to a country where English is rarely understood, it’s almost never needed anymore. I would agree with the other poster that something else was going on and they used it as an excuse. If the agent was fluent in English then he should’ve accepted your license just fine. I would definitely file a complaint with Hertz corporate, but at the same time would expect very little action on their part, since it was a licensee