Developer here. CheerpX for Flash runs the Pepper API version of Flash, and you're correct about the license. However, we don't do full Linux emulation just to run Flash - we emulate PPAPI and run the Flash player in an x86 JIT (CheerpX).

Yuri talks about CXFF's architecture here:

https://youtu.be/7JUs4c99-mo?t=1045

...but if you wanted full Linux system emulation, we got it! https://webvm.io