Earlier quoted context omitted.
I've never understood this obsession with public source repositories. What is the difference between experience writing proprietary code and experience writing code that happens to be published on GitHub? Both kinds of experience are valuable. Most tech employers own everything you do, even on your free time. So a large GitHub history can send the message that you're not employed all that much.
So, here's the deal. I don't think it is fair to expect you to talk about your old company's codebase in an interview--I'd love to have that conversation, but I understand if you wouldn't feel comfortable doing so. So, I can ask you questions in person, and get some response. Maybe you interview well, and you can answer things easily; maybe because you know your shit, maybe because you crammed with "Crack the Coding…
I'm curious as to what your thoughts are on the difference between these two things. Rolling from scratch seems like more work, but sometimes gathering enough knowledge and understanding about someone else's project to be able to make a change to it is even more work.