Good SREs can support a service without being familiar with the details. They use generic mitigations (e.g. rollback to a know good release, request an emergency quota increase, etc) to quickly stop the bleeding without needing to understand the root cause. Later the SWEs who understand the code can debug at their leisure. Having a deep understanding of the service obviously helps but is not essential. I expect agents are already applying these generic mitigations at bleeding edge companies but do not have firsthand experience as an SRE to confirm.