Unfortunately, neither are Open Source Software. The BSL is not FOSS. They're both proprietary.
I can see the source code of Unreal Engine too. Does that make them "further into the open source world" than WarpStream too?
I don't have a horse in this particular race but WarpStream's blog post is a lot more charitable towards the project in question, and the open source world in general, than Redpanda's.