The reaction / reactance may or may not be rude or agressive, but that's just how people react individually to confusion / irritation / cognitive dissonance.
To be honest the story would be much more believable if the writer weren't a foreigner - that kind of reaction from the shop owner is usually reserved for much stronger violations of the "pizza code" (pineapple pizza being the go-to example!)
The only exception is the chocolate, but it's allowed in stuffed pizza only.
Not for all Italians. My paternal grandmother is from northern Italy, but moved to the UK with her new husband shortly after the second world war. My father, growing up with an Italian housewife as mother, experienced a lot of Italian cuisine, but never saw pizza until he went away to university in the mid-60s.
Upon returning home one vacation, he said to his mother "I discovered a wonderful Italian dish at university called pizza... why did you never cook this?" -- only to be met by the scornful reply "Pizza? That's what poor people eat!"
God yes. It's a matter of faith for some that they have the best food, that's all there is to it, and there's no point in having food from other places.