Going by the numbers presented in this article, it's actually Denmark. And our bike culture is mostly a feature of Copenhagen, so really Copenhagen is the global hub.

Cargo bikes were always quite popular in Copenhagen. The dominant brand for several decades was produced in Christiania (a hippie/anarchist commune), so just a few years ago they were all still known as "Christiania bikes" in Denmark. I think the more recent addition of electric assists made them explode in popularity and other brands became more popular.