So what?

Let's say you know nothing and just copy somebody else's code. Good for you. The next step in the interview process is that you have to do a code walkthrough explaining what you did and why. Do you really think someone can get past this stage with a code they just copied from github?

I can even imagine that someone finds this existing code, and then they copy it, then they improve upon it, and present it as such. If they are open about it, I'd have no problems from the interviewing side. In fact, it could even be better, because the more complex the code is, the easier it is to talk about it (and gather information about the candidate).