No, it isn't true, it's perfectly reasonable to add columns to a link table to describe attributes of the link (in this case, the attribute is the time when the link was created, I imagine). If you want to describe dating of links in a many-many relationship (which is a pretty common requirement) this is the right way to do it. Your DDL appears to generate exactly that, so if that's what you want there's no earthly reason not to do it that way.

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f course there are some people around who say that a link table should have only its several foreign keys to identify the objects being linked, and a surrogate primary key - any attributes of teh link have to be consigned to another table which shares that primary key. But saying you have to do it that way is dognmatic nonsense, not relational theory.