The rumor implies that instead of actually buying and holding real Bitcoin, BlackRock is just getting these IOUs, which could artificially increase the supply of Bitcoin on Coinbase without real demand. 

 What would be their incentive to do so? Do they not have the BTC? 

 Maybe to keep Etherium up? There's supposed to be an ETH ETF #thoon.  

 Price manipulation? 

 But how do they benefit from it? They would need to benefit from it for it to make sense if they do not benefit from it in any way, then they would only damage their own business.