What Iris is trying to say is that something so incredibly niche is what's questionable. A bookstore about pets alone would be a bit too specific. One that's just about dogs, in general, would be way too specific. But one that's about specifically dog breeds? That's getting down to the granular level and is the part that's unrealistic and would be considered bad writing. It's the sort of thing you'd see in a cartoon as a joke.
A general bookstore is fine. Even a pets themed bookstore. But once you get deeper than a broad theme, it's unrealistic that such a business would exist. You could make it work if you're going for the absurd angle, but not if you're going for realism.