If its rusting form the back you should be fine, if its just surface rust you will not be covered. That is standard with all rust warranties.

Two years ago I had a protracted dialogue with VW customer services over perfortion rust on a 2005 Passat. It also was a known problem and even had a VAG bulletin on how to fix it but they wouldn't pay for 100% of the repair until I said I had had enough and would prefer to try my chances in the small claims court rather than accept their measly 40% offer which they had improved to 70%. They relented in the end which I knew they had done with lots of other cars as the forums were awash with stories of outright rejection through to 100% paid. Interestingly (to me) it was dealt with as a goodwill settlement from customer services due to my loyalty to the brand (VWs, Skodas and Seats in the family) as the warranty department had said it was not in any way a warranty issue - strange that as the car came with a 12 year anti perforation warranty and had been main dealer serviced so the annual inspections must have been done.