if the winklevoss brothers wanted to sell their stash, they could always do so. why would they go through the most complicated way of an etf that needs approvement, paperwork, etc. if they wanted to sell, they would have done it already otc or via exchanges.
Obviously they expect that the opening of that new market will create a demand for BTC (indirectly, via their fund) greater than their holdings, so they will get a lot more $$ than they would by selling them now.
Which it may. However the SMBIT and PBP funds do not seem to have created much extra demand over their history. Would it make much difference having a fund traded on NASDAQ, instead of privately?
SMBIT started in September 2013 with a stash of ~18'000 BTC, and sold shares that represent ~107'000 BTC, about 2/3 of them before December:
https://bitcointalksearch.org/topic/gbtc-bitcoin-investment-trust-observer-337486 So they sucked up about 90'000 BTC from the "great market" (including OTC). (Apparently they haven't sold any net shares in the last month; which may explain their sudden desire to help the poor Argentinians, instead of the poor Americans.)
Their profits are hard to estimate since they depend on how much they paid for those 107'000 BTC compared to the market price at the time they sold the corresponding shares. However, they should make at least 5 million USD just on fees over those shares. So, even if the SMBIT owners were "dumb" and made no profit from strategic buying, and had to hire a couple of staff to manage the fund, they can sleep soundly and smiling every night, no matter what color stands out on the BTC/USD charts.