Sorry there's an error in that. It should read 'if you're long 10,000 RBS and sell 10,000 of an ATM call, this is identical to being long 5,000 of the stock and short 5,000 each of a call and a put'
Or, identical to being short 10000 put - for amateurs like me, it's easier to visualize the PL like this rather than long half the stock and short half the straddle.
