Turkey's a big military power, and I believe Russia would sincerely like to avoid having an even larger war. But also, Armenia has been turning away from Russia and drawing closer to the West following political changes in the last few years, which will dissuade Russia from coming to their aid too quickly.Why is Russia treating Turkey with kid gloves here? This is their ally under attack.
Russia is laying a line in the sand and saying they will defend Armenia proper if the country is invaded, but are not offering to defend Nagorno-Karabakh. This is an understandable position that is a delicate balancing act.