Self compiling as I understand it, is the ability to compile the OS using only tools on that OS.
For example, if you are compiling to mobile phones generally you have to compile on a desktop PC. That makes sense due to the speed of desktop PCs compared to mobile phones. Compiling would take ages and kill the battery.
However if you had to run Windows to compile Mac OS then that is plain crazy and invalidates any claims of that OS being superior. If it can't compile itself then you are basically admitting its still not a full OS.
