Query and selection results in WAB can be saved to "My Content". They appear as a Feature Layer, but are actually a feature collection. They therefore can't be added to an app using the "Add Data" widget (or in any other way).
Enabling this content to be available in the "Add Data" widget would allow complex queries and selections saved to My Content to be added back in in later sessions, or shared with other users to be added in. Very frustrating not being able to do this, despite being able to save the query.