I agree with your larger point. Though I think it's pretty natural to wonder how the poster ended up with a multi million line codebase.
15 people is a pretty small startup, what if this is a 50-person startup?
Doesn't seem like that much to me, depending on what you're building and the size of your team.
15 devs putting out 400k LOC a year into the same codebase is not normal at all. I’ve never seen anything close to that kind of rate of growth across that number of people.
And 50 engineers is not even remotely close to a small startup. I have worked at a startup that had 50 engineers after a few years, but it was a multi billion dollar unicorn.