"X" didn't split -- it's closer to the truth to say it reunified. XFree86 split. At the same time, the X organisation (formerly a hidebound organisation aimed at proprietary vendors, that hosted an antique X codebase containing almost none of the XFree86 improvements) saw the light and accepted the people who left XFree86 (which was nearly all the important people). The result is today's X.org.
Mac OS X is indeed from before the XFree86 split. But I don't think Apple would have cared to base their windowing system on X, either way. They do supply an X server for those who want it.