Is this legal? I know decompiling is legal in some areas for specific reasons. But I don't think that you can open source the result of that decompilation. You end up with an artifact based on something you don't have the rights to relicense.

Well, its not legal obviously to decompile and share the code online. But its just a matter of "will they care" and the answer is most likely NO. Just like SM64, it is open sourced on GitHub but they never got any complain from Nintendo asking to shut down anything.

https://github.com/phoboslab/wipeout-rewrite hasn't gotten any flack and it's rewritten from the PC leak.