It’s because someone could short (borrow) GBTC and redeem it for BTC for an immediate profit. The premium would disappear near instantaneously as short sellers captured the spread.
Also, I don't see how shorting gives you immediate profit. You still have to give back the stock later (you're in debt). If the stock goes up, you lose. If the stock goes down, you gain. I don't see how it's immediate profit.
1. Use cash to buy Y GBTC at $X 2. Redeem those GBTC for Y bitcoin. 3. Sell those bitcoin for $Z > $X
Short selling isn't necessary, but it would sidestep the "use cash" part of step 1.
This trade is not currently possible because GBTC does not allow people to trade in GBTC shares for the equivalent bitcoin.
But the quote I was quoting was only talking about when it was trading at a premium. That's when I don't see how redemption could be useful.