They didn't write 100k plan lines. The llm did (99.9% of it at least or more). Writing 30k by hand would take weeks if not months. Llms do it in an afternoon.

That's not (or should not be what's happening).

They write a short high level plan (let's say 200 words). The plan asks the agent to write a more detailed implementation plan (written by the LLM, let's say 2000-5000 words).

They read this plan and adjust as needed, even sending it to the agent for re-dos.

Once the implementation plan is done, they ask the agent to write the actual code changes.

Then they review that and ask for fixes, adjustments, etc.

This can be comparable to writing the code yourself but also leaves a detailed trail of what was done and why, which I basically NEVER see in human generated code.

That alone is worth gold, by itself.

And on top of that, if you're using an unknown platform or stack, it's basically a rocket ship. You bootstrap much faster. Of course, stay on top of the architecture, do controlled changes, learn about the platform as you go, etc.