You can read as much or as little of the code as you want.

The status quo was that I have no better understanding of code I haven't touched in a year, or code built by other people. Now I have the option to query the code with AI to bootstrap my understanding to exactly the level necessary.

But you're wrong on every claim about LLM capabilities. You can ask the AI exactly why it decided on a given design. You can ask it what the best options were and why it chose that option. You can ask it for the trade-offs.

In fact, this should be part of your Plan feedback loop before you move to Implementation.

You can ask the AI why, but its answer doesn't come from any kind of genuine reasoning. It doesn't know why it did anything, because it doesn't exist as a sentient being. It just makes something up that sounds good

If you choose to take AI reasoning at face value, you're choosing to accept pretty strong technical debt