The line from Blockstream's own post-mortem that matters: the 11-of-15 multisig worked exactly as designed. The failure was in the software that decides whether a transaction is valid.

When you design these systems you focus on the main shield. Eleven signers, very unlikely to collude, so it should be safe. Meanwhile there's a kink in the armour somewhere else that makes all of it irrelevant.

https://ungovernable.network/shows/bitcoin-brief/did-bitcoin-just-go-dark/