It might take one person one year to write 25k lines.
A year? I've regularly written that much in a month, and sometimes in a week. And, counter-intuitively, its during those sprints when I'm pumping out thousands of lines per day that I write the code that turns out to be the highest quality, requiring the fewest number of bugfixes later. I think it's because that very high productivity level can only happen when you're really in the zone, with the whole system held in your head. And when you have that full context, you make fewer mistakes, because mistakes mostly derive from not understanding the other pieces your code is interacting with.
Of course, that kind of focus is exhausting, and you can't do it long term.
How does a person get their head around that in 15 hours?
By focusing on the structure, not the details. The LLM and the compiler and the formatter will get the low-level details right. Your job is to make sure the structure is correct and maintainable, and that the test suites cover all the bases, and then to scan the code for anomalies that make your antennas twitch, then dig into those and start asking questions -- not of product managers and developers, usually, but of the LLM!
But, yeah, it is challenging -- and also strangely addictive. I haven't worked more than 8 hours per day for years, but I find myself working 10+ hours per day on a regular basis, and then pulling out the laptop in bed at 11 PM to check on the last thing I told the AI to do, mostly because it's exhilarating to be able to get so much done, at such high quality, so quickly.