But sure, this is a step too far
Cuisine is an element of culture and so a socially-defined thing. Few would argue that espresso isn't a part of Italian cuisine, but it was only really invented in the 1930s (coffee had been consumed in Italy for long before that, of course). Ciabatta was invented in 1982 and I'm sure most Italians would be fine with serving that.
But pineapple on pizza is not just not part of the socially-defined Italian cuisine; it's an antithesis to it! It's an emblem to everyone of how you can take something that is part of a cuisine (pizza) and do something to it that is completely at odds with it. Whether you like pineapple on pizza or not, that should be clear.
So, it shouldn't be surprising that some people feel strongly enough about what is and isn't part of their cuisine and draw the line there. In my experience, many Italians do feel very strongly about their cuisine, more so than any other nationality I've talked to about food. Try asking some what they think about spaghetti and meatballs for an example that seems innocuous to most non-Italians.
I respect such viewpoints but I do think there are plenty of people who hold them without really understanding them - who don't realise how malleable cultural elements like cuisine are and who think they're actually upholding an objective and immutable standard. That's clearly wrong, but just because what is part of a cuisine changes doesn't mean everything is part of a cuisine.
For restaurants there's a decision to make - do you stick to what you hold to be the authentic cuisine, or do something more accommodating of globalised expectations? There's room for both... and individual cooks who aren't the restaurant owner or whatever probably don't get to make the call.