I just installed this and it seems really buggy that it's not usable. Not sure if perhaps there is an issue with Catalina?
My Inbox currently has 45 items in it and they don't show up with this workflow. Most of the other sections are blank as well but sometimes an Area will take a few seconds and then populates with Projects and items. Pressing enter on a Project focuses it in Things instead of showing the items within in in the workflow (I believe the docs indicate that that they should show in Alfred), or at least opening the project in Things.
Looks like it could be useful if it worked and was fast.