Everyone here has offered some great advice, and I think most, if not all of us agree, contributing to open source does not show whether or not you are a good coder, but rather what kind of free time you may have :) I've been coding for 15+ years professionally, and longer than that if you include "just for fun", and I've never contributed to open source. Not because I didn't want to, but rather because I know I don't have the time to dedicate to it.
It sounds like you may have a case of Impostor Syndrome . Fear not, this ominous condition may sound scary, but it really means that you are humble enough to know that you DON'T KNOW everything there is about coding. It's not a good or bad thing, it just is. Most, if not all developers experience it early on in their careers.
The good news? It's easily treatable, but does take some work. Just keep doing what you're doing, The more you code, the more confidence you'll get. Be open to other peoples criticism (good or bad) to your work, and don't take it personally, as 99% of the time people are trying to help by pointing out your mistakes.
And as other people have pointed out, it sounds like you may have dodged a bad situation. I wouldn't want to work for some place that would judge my skill set based off of ONE factor such as open source.