Documentation of Future<bool> popRoute() says:

The method should return a boolean [Future] to indicate whether this delegate handles the request. Returning false will cause the entire app to be popped.

While this is true for flat Router structure, it is invalid for nested Router structure.
If popRoute() in NestedRouterDelegate returns false, then entire app is not popped. The ParentRouterDelegate takes charge instead, and on it's popRoute() depends what will happen.