Does this even work? ES was considered 'one work' at some point, right? It's developed together, not file-by-file. How is it possible then to license it file-by-file? Wouldn't most of those files be derivative works of the old 'one work' anyway? (Meaning they have to keep the original license, meaning "the default license, Apache License 2.0"?)

Sure, at some point someone started to create a plugin for ES (let's say the security/ACL thing in x-pack, used to be called Shield or something like that), they used the ES API and they used runtime linking. (I have no idea if that's okay or not, has been tested in court or not. I know the US Supreme Court will say something about that in June.) But when developing any feature in that plugin nobody thinks of just that plugin. Folks think about ES as a whole, indexes, shards, documents, terms, maybe even in terms of low-level Lucene primitives.

I think it's practically impossible to wear the OSS and the proprietary hat at the same time. (Or separately but on the same project.)