Being from Hamburg I've heard that a lot. Too often, really, to not have an ounce of truth to it. OTOH there are barely inhabitants of the town who have any deeper roots there (ie. their grandparents grew up there), so I'd argue that this is not an original cultural quality. "Locals" is a really broad term here, because actual Locals are either gentrified away a long time or if they were able to weather the gentrification, they usually ended up in the posh islets, to which I'd wager you didn't have much contact with.
That's true actually, I know an echte Hamburgerin (3 generations both sides) and she's nice, as are her family. The "locals" I just meant the German inhabitants of the city, many of whom of course did not grow up there or are from there or have strong roots in the city.
Like I said, it's a pity, but for a city that claims to be Germany's "gateway to the world", its people are awfully parochial, small-minded, and terribly lacking in curiosity.