The map you link to is sourced from Ole J. Benedictow's book The Black Death: A Complete History (2004), and it is itself a modernised version of one first published in 1962 by the French historian Elizabeth Carpentier, which as I commented in an earlier thread was largely if inadvertently responsible for spreading the idea that the Black Death spared Poland.

Benedictow highlights the border between Kiev and the lands of the Golden Horde because the disease appears to have spread in a large circle, moving from the Black Sea in 1347 westwards to southern France and Italy, then north, and then finally east - not impacting Christian Russian lands until 1350/53, and not reaching Kiev until that latter year, six years after it was first reported in the Tatar lands of the Crimea. He postulates that lack of trade between the Muslim Horde and the Christian Russian lands may help explain this pattern.

Once again, then, plague maps are showing their ability to confuse and plant false ideas. The lands between Muscovy, Kiev and the Horde were not separated by a long, heavily guarded and patrolled political and military border – such a thing simply did not exist. They were a poorly populated hinterland in which distances were vast and trade was, so far as we can know it, limited.

But the reality, in all these eastern areas, is that detailed records of the sort that do exist for parts of western Europe are entirely lacking, and we also lack any significant understanding of vital matters such as population figures and population densiity. In such circumstances, reconstructions of the passage of the plague have to be done on the basis of scanty, often late, and sometimes entirely imaginary entries from a couple of merchants' accounts and chronicles. Benedictow and Carpentier would be the first to admit that the maps they drew are intended for guidance rather than as hard-and-fast representations of precisely what happened, although Benedictow has gathered sufficient evidence to discuss the problem of the passage of the plague through the lands of the Horde and the Russian principalities over six or seven pages of his book.