Why gorestorm on lifestealer? What is the secret tech?

Aurora vs Tundra match 8370455176

Ingame breakdown
infest: 3.7k
Ancient black dragon 41k (????)

Opendota
infest 3.7k
black dragon fireball 0.1k

Is gorestorm damage not counted or something why is every single pro picking this when you can have so much more control with open wounds refreshing on kills.

Does ingame breakdown just count ALL damage including creeps as well? Why does the Ancient Black Dragon come up as a source of damage he literally uses Fireball once.