It's interesting that Armenian recorded history begins right about the time that Nordtvedt dates the split of their cluster. Another thing is that they began to go into diaspora in the 11th century and became ubiquitous around the Mediterranean as traders, particularly in Italian trading colonies. As the Ottoman Empire expanded and took over those outposts one by one, the Italians were replaced in their trading functions by Jews, Armenians and Greeks.
With that in mind, take a look at the map of territory controlled by the Venetian Republic at its greatest extent:
View attachment 5182
There is uncanny overlap between it and the Mediterranean portion of the I2c map.
So what I am proposing is this: The ancestor of the Armenian cluster settled among the proto-Armenians around the beginning of their recorded history - perhaps as a trader entering the region via the Black Sea. His descendants became preeminent in Armenian society at some point early on and hence expanded their share in the Armenian haplogroup pool. Some members of their cluster became active in trading during the diaspora and consequently spread the haplogroup around the eastern Mediterranean, particularly in Italian trading colonies.
Armenians are also known to have established trading colonies in Russia and the Ukraine early on (beginning in Lvov in the 13th century), and later in Northern Europe, especially in Amsterdam. This might account for the lower but still measurable presence of their cluster in those areas.