As described in the article, sharing data between apps is currently impossible. I wish Solid/PODs had taken off, but I get the sense that project spent more time on ontologies and less on making useful things.
How can we draw apps into using a common data backend owned by the user?
Almost all apps I use can export their data into common and open formats. If you make your workflow around common file formats instead of specific apps, then you can easier share data between apps.