in a bitcoin spot ETF .. they do not trade in bitcoin...
the idea actually is they manage, as agents of the ETF a basket of SHARES which are sold for fiat

its already been known that the SEC wants to know which btc exchanges will secure, audit and transfer the actual bitcoin (EG  coinbase for greyscale)

si lets say broker-dealer "ABC investments" wanted to be spot ETF agents..
they would with fiat, go to coinbase. pay $XXXXX fiat for a basket worths of BTC and coinbase would custody it and register the basket with greyscale. grayscale will give ABC investments a lump of GBTC shares. which ABC investments can sell to its customers

ABC investments would not be trading bitcoin with its customers

m sure the link your wrote got details in wrong order. but im thinking the SEC is talking to coinbase to ensure coinbases systems are ready to register the baskets with blackrock, greyscale, ark.. so that agents can hand fiat over and the ETF hand shares back in a secure audited way