In general terms, it isn’t really. Or at least it is a controversial topic still subject to discussion.

https://opensource.org/blog/public-domain-is-not-open-source

As to this specific topic, the fact that all of the code has to be open source is part of the 10 criteria https://en.wikipedia.org/wiki/The_Open_Source_Definition

As such, you can’t consider open source the public domain portions of a codebase that also has proprietary portions.