Another reason postmortems can be vague is that, for each detail you add, you might need to add even more background details. Then it becomes a recursive problem.

In this case, the "configuration change" could be a feature flag in the L2 GFEs, something in the L1 GFEs that changed L1->L2 requests in minor ways, or maybe something else entirely (since it's partially security-related: a dynamic LOAS handshake change to use different cyphers? So many possibilities). At the end of the day, though, it's still a specific permutation of all possible features and flags that hadn't been vetted before. Given how large Google's monorepo is, it's not impossible for one of your many dependencies, even indirect ones, which might be configured by another team entirely, to have subtle time bombs that only trigger well after you have built and deployed the code.

Having been on the other side, I know that, for every detail added, a bunch more questions come up.