I don't think this 15% number matters much when it's such a tiny number of people. For all we know half of those could be people who were only in the city on a somewhat temporary basis while the census was conducted, e.g. some business person who is living there for 3 months while doing a job who intends to go back to Ontario after.
There isn't necessarily anything wrong with ethnic identities. They can be purely descriptive with no sense of judgement even if some people are in some groups and not others.
Most of the time we talk about them and they have no particular positive or negative connotation. For example some people say they are Italian (people living in NJ who have never been to Italy). If you had no Italian background whatsoever, moved to Italy and then 1 month later flew to Toronto, would you say "I'm Italian" to people in Toronto who ask? Is it because of some huge morally charged problem one way or the other? Probably not. It just wouldn't be an apt description and would confuse the other person.
One thing I noticed long ago in BC is people are more likely to have a notion of what their "background" is even if they were born and raised here and barely participate in the other culture(s). It's somewhat less common in NS, where more people have a complicated ancestry within the region, although I wonder if it's becoming more common now that there's so much more immigrant and more people seem to be moving there from elsewhere (previously "away"
).
I'm 50% "Québécois de souche" ancestry-wise but it would be weird for me to identify myself as Québécois or "half" Québécois. I think it's just one of those strange terms that's currently a mix of background, culture, and place of residence. Also note that Acadian is not "from the Maritimes + Francophone ancestry", it's a specific group tracing roots back to specific settlers. It's not a particularly high status grouping based around excluding people, it was a way to describe the historical cultural group.