It's my understanding that Oracle Synonyms are just pointers? So Designer importer recognizes that it's of type "Base Table" but does not follow the reference to the actual table to retrieve column data. The error you get is a validation error. Tables must have at lease 1 column to be valid.
Is there any way of making Teiid following the sysnonyms?
Tools like SquirrelSQL does follow them and sometimes access right allow just that privilege?