yes they dont sell coin, they just hold it. but that means there is a inflow gain and not much outflow loss of hoarding
once blackrock sell their creation of shares to secondary managers like robinhood/etoro.. blackrock BUY more coin to create more shares
this means coins have to come from somewhere
when people pay into a pension they are buying and hoarding shares of something. so each month new shares are needed or traded.. thus as long as the demand for shares keeps up the demand for coin keeps up
They become like a stabilizing force, they wouldn't add to the sell pressure during a Bear but would add to the buy pressure during a Bull market as more investor would want to get Bitcoin through ETF.
normally smart people buy the bear(dip) and sell the bull(pump)
you know ,, buy low sell high
however ETF do buy low they wont be doing much selling, they instead are a net flow of buy and hoard (inflow and accumulate)