The stated routes for that fare are either via Elephant & Castle (EPH), or via Clapham North (CPN) & High Street (CLP).
Changing at Clapham Junction (CLR) and Peckham Rye (PMR) would seem to make a lot more sense to me - but as the default route is charged via Zone 1 and you wouldn't pass through any barriers when changing at CLJ & PMR, I don't think you'd be able to take advantage of the 'avoiding Zone 1' fare if you went that way.
I suspect you're therefore forced to take the slower route with an OSI at EPH or CPN & CLP to take advantage of the cheaper 'avoiding Zone 1' fare. Ironically I think you might even be charged the 'avoiding Zone 1' fare if you do an OSI at both EPH and London Bridge (LBG) - despite that involving travel via Zone 1. Perhaps
@MikeWh can clarify.