From this comment on the PR for CDRIVER-590:

Previously, such a URI would result in the client's read preference having tags with the primary mode, which is invalid according to the read preference spec and mongoc_read_prefs_is_valid(). That said, mongoc_read_prefs_is_valid() was never actually called anywhere, so I'm not sure how libmongoc handled such a structure.

So, although mongoc_read_prefs_is_valid() exists, it's never called anywhere (apart from the URI validation added in CDRIVER-590). If we compare this to write concerns, those are checked before any write operation and can yield an error before we send anything to the server. I expect any read operation using a read preference should do the same and yield an error before attempting server selection.