This was tried by apple. https://www.engadget.com/apple-drops-its-lawsuit-against-a-m...
An open source client for an API need not include any non-original works.
It targets games, so manages to be useful without having to emulate or re-implement the majority of the OS.
The situation seems very similar to the AACS key leak back in the day: https://en.wikipedia.org/wiki/AACS_encryption_key_controvers...