Living in New York City, the most walkable city in the US I believe, was the unhappiest I've been in my life, and I know plenty of others who feel the same way. So, there's that.
IMO, "walkable city" means not just that you can technically walk to a large number of places, but that you can do so relatively unimpeded.
I suspect Manhattan (and the densest parts of the other boroughs) have enough density, business, noise to offset the benefits of being walkable.
But, I'd guess there are plenty of residential neighborhoods in NYC that are close to being "15 min cities" on their own.