Don’t think that’s the way it would work. They hold Bitcoin, they don’t run nodes or miners. If there’s a hard fork, they would hold Bitcoin and the equivalent units of the forked coin. What they could do (which would still be very damaging) is dump real Bitcoin, reduce the price significantly and hold the forked coin to represent the ETF capital. Eventually investors could redeem for cash, buy real Bitcoin and eventually it would recover. But it would be painful.