You need an IDP in Austria. It only takes a few minutes and a few dollars at AAA. If you cannot get to an AAA office, I noticed a while back that on their web site they do have a way you can get one by mail. No doubt the mail process is much more inefficient than stopping. A few years ago I contacted the Austrian Tourist Office on this subject and their reply included the phrase, "only a fool would drive in Austria without an IDP because the financial liabilities are unlimited, if you have an accident."
The people at the rental desks are not the police. They are paid to rent cars and since they have YOU by the CREDIT CARD they are not the least but worried about the costs of retrieving an impounded vehicle ... they will be paid.
We lease, and the leasing companies are more stringent on honoring the law. On our first trip to Italy when we picked up our ca at the Malpensa Airport, the guy was very straight forward: No IDP, no car and there would have been a $750 fee for last minute lease cancellation. ( The people in at CDG are much more subtle about it.) Fortunately I had paid no attention to Rick's chatty line about it may be the letter of the law but he has driven lots of time without one ... funny how a 'journalist' can encourage a reader to break the law.