Can there be content sent the a vanilla Drupal 8 sites' views that isn't a node?
You don't import things into a View; you configure a View with certain criteria, and that basically runs a query against the database, the results of which are formatted and rendered according to the rest of the View config.
It's not restricted to nodes, many entity types and pseudo-entity types can be queried, as Views is very extensible.
You mention importing xml into views - in reality that happens by 2 separate processes - importing XML and creating content/entities from it, then setting up a View to display that content.
Don't think of a View as a container. It's essentially a sophisticated HTML report builder that knows how to get and format Drupal data.