Well XNU has always been 'open-source' [1] for some time on Intel machines but its just that ARM support remained closed (until 2017) which is why the darwin-on-arm [2] project existed. But I'm assuming that the OP means that the ARM64 code was recently open-sourced in the XNU sources[3] making a possible open-source ARM port much simpler. I still find it quite interesting to see some regained attempts in emulating iOS; with Corellium being the furthest at doing this.