It is open source. Many companies which contribute to it are American, but nobody from America can tell you what you can or cannot do with it - unlike Microsoft or Apple with their proprietary OS being forced by US government.
So Open source it may be , however there are still pressure points that can be used. I believe this is one of the main reasons RISCV foundation moved to Europe.
Even if upstream linux banned european contributors, there are enough european contributors that a fork would just emerge. So I’m really not too worried about that happening.