I think most people use their ira/roth ira on fidelity to buy GBTC. I don't think they can buy COINXBT on fidelity if it's an IRA? At least I cannot see it and I only have Stocks/ETF available. So the big part of GBTC are IRA's.
You can do it, and it has been done. Quoting from post 2118:
Details at Fidelity are as follows. Taxable accounts can execute online, the Fidelity-specific symbols are BITCOIN_XBT:SE (trading in Sweden, in krona) and BITCOIN_XBTE:SE (trading in Germany, in euro). For an IRA, the trade can only be placed with a human, not online, by calling their international desk at 800-544-2976, so the commission is higher than for online trades in a taxable account. To trade the ETNs online at Fidelity, make sure your account is enabled for international trading, then hover over "Accounts & Trade" until you get the dropdown > select "Trade" > click the "International" tab > then enter one of the Fidelity-specific symbols such as "BITCOIN_XBT:SE".