My gut feeling is that any serious real-world company with a proprietary codebase worth looking at would not be handing out the crown jewels to a third party. License or not.
I don't doubt somebody licensed their codebase to them, I just have my doubts about who the "who" could be.
At any rate, I'm not sure it matters whose codebase it is. I'd even say that a shitty codebase might make for a better test.