Could it be that Tumblr handles are actually owned by Tumblr, thus it is Tumblr which was "infringing on trademark" and needed to act on Demand Letter? I would

Could it be that Tumblr handles are actually owned by Tumblr, thus it is Tumblr which was "infringing on trademark" and needed to act on Demand Letter? I would love if you or somebody else with more legal knowledge than me (which is not hard :)) can explain this.