Unarguably it’s a global phenomenon. And personally I don’t greatly value authenticity - or adherence to a particularly canonical cuisine - but changes to a cuisine’s canon don’t just happen due to time passed; that was really my point with the ciabatta example, that it is accepted in spite of being very recent. I haven’t asked anyone explicitly about ciabatta, but I would guess that even though it happened recently, it’d be accepted by most Italians, even those who are fairly strict about what counts as authentic, as Italian cuisine.

Why, even though it’s so recent? Well, in the end it’s about social agreement, not rules, but there are probably some reasons that go into that agreement: it was developed in Italy; it has similarities to existing cuisine; those similarities were consciously developed; it doesn’t violate any implicit expectations in the existing cuisine. Hawaiian pizza certainly doesn’t share those aspects. That doesn’t mean that it couldn’t be accepted, but it means it’s harder.

To take another example, sandwiches are an accepted part of British cuisine, and have also well and truly transcended those roots and become global. But that doesn’t mean that banh mi are part of British cuisine!