They can fund those buffers any way they want, but those are just the buffers to make sure they don't go insolvent. All that money is in addition to all the BTC they sell to other parties, it doesn't include any of the demand that comes in from the actual ETF. Again: it's just an "oh shit" buffer to provide emergency liquidity. It's called "seed money" for a reason. It doesn't represent their actual liability.