A knockdown prophecy

[Angrboda]

It's nice the writer was able to accurately count joints, but that's early science, not prophecy. The writer wasn't ever the first to arrive at an accurate count:

Lü Pu-wei's Springs and Autumns (Lü shih ch'un-ch'iu) - of (ca. 239 BC)

Human beings have 360 joints, nine body openings, and five yin and six yang systems of function. In the flesh tightness is desirable; in the blood vessels (hsueh-mo free flow is desirable; in the sinews and bones solidity is desirable; in the operations of the heart and mind harmony is desirable; in the essential ch'i regular motion is desirable. When [these desiderata] are realized, illness has nowhere to abide, and there is nothing from which pathology can develop. When illness lasts and pathology develops, it is because the essential ch'i has become static.

Edit: Props to Helios for ninja-ing me on this.

Also, it's known China exported its medical knowledge beyond its borders. So there is no reason it couldn't have reached the Arab world and become known to its populous.

And one can't overlook the possibility that, even if true, Mohammed simply got lucky by picking the right source.