I don't know if I'm right or not but I heard somewhere that Coinbase's most Bitcoin's lie in their offline cold wallets that no one can hack unless they get to it physically get access to it.
Which is completely possible. There have been entire banks robbed in the past. It'd be completely feasible for the same thing to occur in Coinbase. (And I highly doubt all their assets sit on airgapped wallets.)
Unless they have a SAFU fund like Binance and publish their audits on their website for everyone to see (unlike Binance [until very recently]), you should always assume that they are not insured.
How's Binance different? It does have as well most of its assets in crypto. If they're compromised, they cannot pay their clients back, just as with Coinbase.