If you don't understand how your system works, your ability to make good decisions about future work on that system quickly degrades.

I don't think you need to review every line of code, but you absolutely do need to be able to describe how the system works and its high level structure.

As is so often the case with coding agents, having experience as a tech lead or engineering manager really helps here. You are responsible for a large system that has been worked on by multiple different collaborator (both human and agentic). You need to be able to make smart, informed decisions about that system, and talk with credibility to other stakeholders about what it can and cannot do and sensible next steps for the project.