The model is called Open Core, so is well understood so I am not sure what is causing this confusion. The editor is open source as evident from a dozen forks. The complete experience which includes extensions has closed source pieces which the forks won't access to. But OSS community can build replacement or other companies can provide alternatives.

Just because pylance is available doesn't stop jetbrains/Google/OSS from creating an LS. Maybe no such exists as if now, but not from a technical blocker. Just no one created one.