Web pages use a data source behind the scenes, but web pages are NOT a data source. A rendered Qt app is not a data source either. weboob builds plain-old objects (or JSON), it is a data source. The Qt applications are just an example front-end using weboob data source as a data source. Furthermore, this data source is made for aggregation/standardization, as weboob returns results in the same format whatever the site you choose, so it's still better than if each site proposed an API specific to the site itself.
Deliver the QML over the wire here and you just reinvented a kind of browser, that consumes html|json data from the web and has a different native UI library.