This is a good point, and one I forgot to address.
We would never ask to see proprietary source code. Hopefully, a candidate would have a side project or open source work they're willing to share. If not, we'd definitely work around this. For team projects, we'd ask them to describe their contribution. So far, this hasn't been a big problem, but we're still iterating and making the process better!
The article is definitely not clear about this. In fact it says "bonus points if it's open source" implying that you expect to see proprietary work. It was a bit of a red flag for me while reading.
I interpreted that as "bonus points if it's contributions to a big official Open Source project, as opposed to your personal pet project with no exposure or users".
We would never ask to see proprietary source code. Hopefully, a candidate would have a side project or open source work they're willing to share. If not, we'd definitely work around this. For team projects, we'd ask them to describe their contribution. So far, this hasn't been a big problem, but we're still iterating and making the process better!