If its is their own code it shouldn't be a problem.
If it isn't their code and they can still talk you through it, even better - they have managed to understand code that someone else wrote which is probably an even more valuable skill.
And no, having somebody who's an energetic fraud on the team is poisonous. Having somebody who's so good at it that they're hard to catch is worse, not better.
I didn't write the monstrosity that I am working on just now, but I could explain how it works and I know where to look to fix bugs. I don't know why some crappy design decisions were made. Does that make me a worse programmer than if I had written it myself? Is harder to understand someone else's code than stuff you have written yourself. There are far more jobs working on existing code bases than on new new projects.
(My guess is that you have never worked with anyone who has claimed ownership of someone else's code, have you?)
On the other hand, fakers have caught on to the "GitHub is my resume" thing. I have had applicants who have basically fraudulent Github projects, or who have taken group projects on which they did basically nothing and claimed them as their own. So a Github page now requires an expert to evaluate it.