It is not enough to be able to authenticate a user based on its user/password (or whatever other authentication parameters you use), as user capabilities will be tested all over the code, both on admin and front end, and maybe other user based information like email address, avatar etc.
Therefore you need to simulate all the relevant information, and frankly, the easiest path is to just create the users as proper wordpress users even if you do very little with that information for the users you are interested in.
Or to say it differently, you get the error because your authentication do not return WP_User
object. You can create a class inheriting it (maybe), but the effort will probably be much bigger than creating a proper user in the DB.
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