Can the ETF companies stack BTC?

I’m not familiar with the way the latest BTC ETF companies have decided to play the coming bull run.

Obviously they have to hold actual BTC to match the incoming funds of customers investing in their ETF. That’s plainly clear.

But can they stack excess BTC prior to the bull run and sell at the top to rake in extra profit for themselves?

I’m from the UK and I’m not sure how the US financial system has regulated these companies. Are they allowed to stack their own BTC?

If they can (and I am assuming they can) then this could control a lot of the forthcoming price action. Yes?