Difference between DataLinks and External tables
If I remember correctly, Data Links Manager attempted to manage external files that were referenced in a database, rather than providing relational access to the data in those files. Think of an image or PDF document that would be related to a record. You could store those documents in the filesystem (instead of in a LOB inside the database). The files could be accessed outside of the database, but Db2 would also (supposedly) provide some control to prevent files from being deleted or moved. 

It was, perhaps, closer in function to products like Content Manager OnDemand or Filenet.