Well, while that's true in theory, it's not exactly true in practice. Funding and borrow costs differ across the platforms, while if there is an exchange-traded product it's far more transparent. Also, futures provide long/short symmetry with regards to borrow and funding costs - right now, if you are long a HTB stock, you get no benefit while your broker is getting rich.
Doesn't IB brag that pay you those high borrow rates when a stock is hard to borrow so they can lend your shares?
I am not using IB, for obvious reasons. For brokers that do some pass-through, in most cases the “bid-ask” on these rates (borrow vs rebate) is like half of the carry. I am sure IB is not an exception.