NRC Handelsblad said multiple sources confirmed to them that Dekker was Clasicomano Luigi. It's a good newspaper, they wouldn't likely make a simple mistake, although sources can always be wrong.
Calling Dekker a classic specialist wouldn't fit him in 2006. But then again, neither would Cancellara really. At the beginning of 2006 he had finished 4th and 8th in Paris-Roubaix, good results, but I don't think anybody considered him a real specialist in the classics back then. He was mostly known as a prologue-specialist.
What was strange about Cancellara (and Dekker) is that they both rode terribly at the 2006 Tour de Suisse, and were both removed last minute from the Tour-team because of their lack of shape. Panic and a lack of blood could explain why Cancellara was suddenly so bad at that time.