Why was I asked for my drivers licence when I am already an existing customer? Will that chat salesman sell my information?
When I recieved the chat transcript my DL was not "xxx" out. All my other personal information was. Is this a concern?
Of late Optus and other Telcos have added extra questions to the ID process as it really was far too easy to pretend to be someone else using the basic info (DOB, Account No and Full Name). As with many things Optus does this is a bit add hoc and manual so while the usual ID elements would be auto xxx, unusual ones (like the licence wouldn't). Note the CS agent doesn't edit your transcript, that is all done automatically. The CS Agent would also already have your licence number on their screen (presumably you provided it when applying for the credit check?).
I'd say your experience was a case of more safety, not less.
Peter has summed it up pretty well, it's definitely for your security.
More information on Optus ID requirements here.