No, the SIPC, like the FDIC, can step in and make people whole if assets go missing. FDIC's for deposit accounts, SIPC is the equivalent for brokerage assets.
"SIPC oversees the liquidation of member firms that close when the firm is bankrupt or in financial trouble, and customer assets are missing."
In Coinbase's case, holding in a custodian account doesn't save them if the Bitcoin in it gets sent to an attacker's wallet. They're just gone, and neither the SIPC nor FDIC cover that asset class.
https://www.sipc.org/about-sipc/sipc-mission
"SIPC oversees the liquidation of member firms that close when the firm is bankrupt or in financial trouble, and customer assets are missing."
In Coinbase's case, holding in a custodian account doesn't save them if the Bitcoin in it gets sent to an attacker's wallet. They're just gone, and neither the SIPC nor FDIC cover that asset class.