Actually the bolded is untrue, just as all derivative markets DO affect the overall market cap and remove the scarcity aspect of the underlying asset.
But the bank that holds no bitcoin reserves does not interact with the bitcoin market, and thus, when it sells a "wrapped bitcoin", it doesn't suppress the price in any way. Whoever takes the promise just plays against the house.
If I am looking to buy something like gold lets say and instead I buy a gold certificate then the gold I was going to buy never hits the market and increases that cap, this is no different.