MANY_MANY relationships in plugins
Craft won't create the join table for you when specifying a MANY_MANY relationship.  You'll still need to manually create a record for that join table.  See Craft's own craft/app/records/UserGroup_UserRecord.php as an example.

What it will do, as mentioned in the answer you linked to is keep the data up-to-date in the join table without having to specifically reference it in your queries.