I think the point here that Vladimir is making is that even the heathens were given wisdom to follow the Ten Commandments (number 6 is very close to the 4th Commandment), and I suppose it is reassuring that God hasn't abandoned the Orientals entirely in their false religions, it just glorifies God's infinite mercy all the more.
Also, it's just another reason to be all the more cautious, in that one has to remember that heresies and other religions are the strongest that are closest to the truth, but distort it in some fundamental manner that one may not even notice.
In a more anthropological sense, it also shows that God has ingrained into man (as a whole) the natural inclination to seek Him out wherever he can be found, and to follow his Law (at least in the most elementary manner).