It is a MVP of 70-80k lines of well written code. If this is true and the app is good, he is ahead of 90% of even the projects accepted by YC which by the last ones I have see the github, it is nothing impressive at all, just React/NextJS.
I assume that YC is looking for people who have validated the market and have established that a market exists before sinking time and effort on the order of years into a product with no customers.
Code is relatively easy against the task of meeting with customers and finding the right fit for your product.
I say this as a coder who much more prefers the scratching of a technical itch to the thought of finding a customer who will pay.