Maybe there would be contexts when it would mean that, but not here. Actually using "really" would make sense only if you changed the sentence into a question caused by your surprise - that is my feel.
The Hover Hints are really confusing here: apparently na pewno can mean Surely... (as given above) - or Are you sure... ?, which is practically the opposite (or maybe just incorrect?).
If I heard. "Surely you do not know this band" I would think it was a question, probably asked because of "surprise that you knew this band" and wanted confirmation. If I wanted to emphasise the "not knowing" I would say Definately, you do not know this band. Definately in this instance meaning - I am sure Almost every sentence that I can think of begining with "surely" is a question.