The rumor is that Coinbase isn't actually buying the bitcoin for the ETFs, but instead borrowing it.
This borrowing settles the ETFs bitcoin balances but doesn't actually put any buying into the market.
The rumor is being fueled by this:
(Source: https://x.com/sashahodler/status/1800258507884663096/photo/1)
As well as the fact that the price of bitcoin isn't really going up much despite tons of bitcoin flowing into the ETFs.
However, I highly doubt this is really happening.
There is another rumor that funds are buying the ETFs and selling the futures.
That would also explain the heavy btc flow and no price action movement.